Estimating an enemy force and the strength balance between your forces against theirs takes experiance and careful analysis of many variables.  There are two main opportunities to gain insight into their force, preliminary assessment and engagement assessment.  Most of the time when meeting a freep force for the first time or when it's first spotted, a preliminary assessment is done to estimate if that force is attackable.  Once battle is joined, leadership should be doing an ongoing engagement assessment

Preliminary Assessment

At this stage the information is gathered earlier by scouts, usually wargs, or by a creep who happens to be in the wrong place and the wrong time and is hit by a train of freeps.  The important information to gather and report in includes the following

• Approximate number of how many (and don't say 'a raid' when there's twelve of them.)
• Location and direction that they are going
• Number of minstrels
• Rough assessment of if they are more ranged or melee DPS
• Amount of crowd control (loremasters/burglers) in the group

Optionally but useful is the following information, this generally requires a veteran creep to report this information

• Proportion of veteran freeps to new/low ranked freeps
• Kinship group or PuG (pick up group)
• Who is the likely leader

If observing them attacking NPCs, also an estimate of damage per second can help assess the strength of the freep force.  In general 3k damage per second is generally the output of a PuG raid's worth of freeps.  5k/s is that of either a raid and a half PuG or a raid of veteran freeps.  Breaking 6k/s is generally a lot of freeps and DPS classes that should not be engaged with less than a raid and a half of creeps strongly coordinated together.

Preliminary assessment information is used to decide where to fight, either in the field intercepting them or in a friendly keep.  Sometimes the decision is moot if the freeps have already arrived to assault a keep.  In either case, engagement assessment begins when battle is joined

Engagement Assessment

Leadership and veteran creeps should be constantly assessing both the creep and freep forces as the battle progresses.  Creep leadership must always be determining when it is prudent to fall back or when to call a full charge forward to break the freeps.  Feeding into that determination are the following factors

• Freep focus fire:  If the freeps are highly disciplined, they will frequently have extremely strong focus fire.  This type of group is less likely to break with pressure.
• Non-Minstrel Defensive Quality:  Keep track of the amount of defense being done.  If there are many captains firing In Harms Way, this could be a very tough fight to sustain (but it is best to keep burning IHW anyways so that it and Last Stand are down for at least 15 minutes). 
• Minstrel Defensive Quality:  If there are a significant number of minstrels and they are cross healing the entire raid, consider splitting wargs off to silence/stun them all simultaneously.  This must be balanced against a melee heavy freep force which may crush individual wargs.
• Ranged or Melee fight:  Ranged freep forces should be taken at point blank range while melee freep forces should be kept away from tight areas at all costs.  If melee forces are heavily present, look to kiting them while hitting their minstrels/loremasters/hunters who aren't chasing creeps around.
• Morale:  Watching how cagey each side plays is a valuable indicator of strength.  If the freeps have been beaten or lost several keeps, they are likely to play more conservatively.  Use this to your advantage by ruthlessly pushing the aggression level higher.  On the other hand, if the freeps have won the last several fights, consider them about 10% stronger than they appear to account for the fact that they will more likely stay in the fight.

Force Balance:  While the above factors (including tactics) are very important, feeling the breaking point of either your raid or theirs is critical.  For creeps, if everyone seems to be reeling back and you have generally blown all of your rezzes, it is probably a good time to regroup.  For freeps, watch the number of kills.  It is very valuable to have an entire chat screen dedicated only to deaths and rewards.  This way you can watch and keep track of what has died on the freep side and how many in combat rezzes have been fired.  Roughly around 25% losses on the freeps (after rezzes) a strong push that sends every creep to point blank range (including BA's) will break a PuG while most veteran groups will break at 33% losses.  Having all of their minstrels dead and all rezzes used up is also an excellent time to break them.  Or if not all are dead but only one or two remain, quickly strike at all of the hunters and other squishies and get them to DF/die to rapidly cause a chain of deaths to break freep morale.

The counter to a team breaking is extremely strong leadership.  Forceful and direct instructions by known leaders can keep a raid together no matter what the losses are.  There are times when a scatter order should not be given, example being the last 10k on a CG to flip a keep.  Know when to stay and when to leave.  Another use of the infamy/death window is to measure the kill/loss ratio.  In general Creeps can win at 3 losses for every freep death (1-3). A worse ratio should call for a pulling back to friendly NPCs while a positive ratio (greater than 1-1) should call for a charge. 

Force Balance equation

Very roughly here are my thoughts on Elendilmir on how to calculate a force balance for either side.  These are meant as guidelines and you should not be sitting there with a calculator working this out.

Freeps = # Freeps remaining + the following

• +10% for each minstrel
• +15% for strong leadership
• +15% for a veteran core
• +5% for each CC class
• +10% if they have been winning the last few fights
• -15% if they've been losing the last few fights
• -15% for poor focus fire
• +5% for a kinship group
• -5% for each in combat rez burned
• -20% if freep losses have hit 25%

Creeps = 90% * # Creeps remaining + the following

• +5% for each warleader
• +15% for strong leadership
• +15% for a veteran core
• +10% if they have been winning the last few fights
• -10% if they've been losing the last few fights
• -20% for poor focus fire
• -20% all WL rezzes burned
• -20% if creep losses have hit 50%
• -15% for fighting in a blue keep
• +10 creeps each troll on the battlefield
• -50% for a dead troll

My numbers come from the following thoughts.  Creeps are more used to death and are less likely to break especially with AoE rezzes available.  Freeps on the other hand are far less willing to die and will break more easily if you start toppling them over.  Also creeps are far more vulnerable to NPCs than freeps are with less aggro and crowd control capability.  Long induction defiler and warleader heals are more vulnerable than minstrels to being interrupted.  Also a troll has significantly more morale breaking ability than a ranger does due to its melee tactics.  It's big, it's visible, and it's bashing freeps in the face.  On the other hand, when a troll dies, morale frequently shatters.

Your thoughts are welcome on other contributing factors!