Several major components contribute to an army's Combat Rating.
Victory of one army other the other is determined by comparing the totals of their respective Combat Ratings, with a small random variable.
Relevant skills can be used to qualitatively evaluate the probability of victory in a given battle, assuming sufficient information is available.

Troop Class
Widely determined by veteran and combat proficiency.
Conscript: Untrained, poorly equipped dregs and other rabble unsuitable for combat. EXAMPLE Penal Legionaries, Peasant Uprisings, or Slaves
Trained: Have some training and conditioning, but are untested in the field. EXAMPLE Imperial Guard Whiteshields, Cultists, Voidship Crewmen
Regular: Possess some measure of field experience and effectiveness, either as a result of lavish equipment or a significant weening process. EXAMPLE Imperial Guardsmen, Noble Household Troops, Navy Armsmen
Veteran: Forged in battle through years of combat, these units have hard won experience in the Crucible of War. EXAMPLE Imperial Storm Troopers, Skitarii, Arbites Heavy Response Units
Elite: The best troops that can be amassed by an advanced race, combining training, veterancy, and more esoteric enhancements. EXAMPLE Space Marines, Enhanced Mercenary Companies, Lesser Deamons of Chaos

Command
While a veteran unit on its own is an effective blunt object, in the hands of an equally veteran commander it becomes a precise instrument. Leadership provides a bonus only up to one level higher than the Troop Class of the unit, as even the most genius of commanders cannot properly convey their wisdom through hopeless subordinates.

Morale
Willingness of a unit to fight a battle. Can range from individuals psychologically conditioned from birth protecting their home, to thankless masses of slaves sent to fight on a toxic battlefield.

Combined Arms and Support Weapons
Units with a diverse assortment of equipment can handle a wider range of situations effectively and can cooperate for easier victory. Can range from infantry equipped with nothing but las-rifles to Battlegroups containing a mix of Infantry, Vehicles, Tanks, and Artillery.

Scouting
A unit able to see and predict the movements of the enemy has a decisive advantage over them. This includes conventional scouting, intercepting and decrypting enemy signals, or technological surveillance, as well as more esoteric activities, such as Divination

Unusual Equipment
Unusual or Unique Equipment that can provide a decisive advantage in a specific situation. Examples include Siege Weapons bombarding a fortress, Air-Dropped infantry landing behind an enemy, or Night Vision capable troops fighting in eternal darkness

Air Support
Units strongly supported from the air gain an advantage over their foes, unless fighting underground or in areas with heavy cover. Also includes Orbital Bombardment Support, assuming such strikes can be made with swiftness and precision. Can be partially or fully negated by strong enemy Anti-Air or Anti-Orbit Weapons.

Weapon/Armour Quality and Reliability
Even the bravest soldier is useless if his weapon cannot damage an enemy. Similarly, a unit aware his armour is largely impervious to the enemy is likely to have little need for bravery. Units equipped in part or entirely with special weapons, such as Plasma Guns, Grenade Launchers, Melta Guns, and Flamers, receive a bonus based on how plentiful such weapons are and who they are fighting. Similarly, heavy armour provides a bonus to a unit, though this bonus can be negated against weapons that can easily penetrate it. Especially poor, unreliable, or badly supplied equipment may impose a penalty.

The Monstrous and the Unassailable
Some enemies are simply without peer among the ranks of the common soldiery. Even the bravest soldier is useless against a rampaging Squiggith or advancing Battle Titan. Such units, especially in plentiful numbers, can heavily weigh odds in their side's favour. At my discretion, such units may simply be undefeatable by their adversaries. What hope does the rank and file have of surviving a blow from a Greater Deamon? Without the most powerful of weapons (or the boldest and most cunning of plans to destroy such a monster personally) such units can advance unopposed. Consider offering a Prayer to the Emperor before your end is upon you.

The Arcane, the Dark Arts, and Lost Powers
Witchery has always been feared and reviled in the Imperium, for though it can draw vast powers from the Warp, it can also call attention from it's denizens. Alas, in the face of enemies who call upon such powers readily, what choice does humanity have but to respond in kind? A creative or powerful psyker can influence a battle in any number of ways, from storming the front as Walking Artillery, to supporting from the rear with more esoteric powers. A psyker or group of psykers can provide a bonus directly to Combat Rating, or indirectly by influencing any other category appropriate to their powers. Similarly, there are near infinite Techno-Arcane devices capable of unique and powerful effects.

Situational Modifiers
There is a near infinity of situational influences on a given battle, from terrain, to poor atmosphere, to distrust within a unit. This bonus or penalty relates to any non-persistent but still relevant battle conditions not covered above. Remember to never underestimate an enemy fighting to protect their home.