How can you estimate the chances to win a war?

1. Estimate your armies: look at army screen where you raise your own and vassal's levies - look for "unraised" - it is in the middle. Add these 2 numbers and add your current retinue. This will give you the estimation of your forces.

2. Open Ledger screen (a button with bar diagramms in the left-bottom), go to page 10 - independent states. This can give you an estimation of your opponent's armies. But it may be inaccurate!

3. Always check for allies using diplomacy view and character view, allies tab.

You may be quite sure to win a war if you have a 3-5 times more potential troops then your opponent.

These factors drive the chances of winning to your side:

• Opponent is involved in other wars. IMPORTANT: these wars should contest other titles, not your target title. Or every member of that war would be hostile to you.
• Opponent is in civil war - there is some rebellion going. You just have to avoid fighting with rebels.
• Opponent's armies are depleted after previous war - quite hard to notice, but you may deduce that fact based on the event on the map
• The contested title is far away from enemy's capitol, ideally - separated by sea. It would be hard to deliver the entire army and you will defend and use terrain bonus, because the warscore ticking would make the opponent to attack you.
• You have a strong fleet and outnumber the opponent at least 2:1 and you can drive the battle to the coastal provinces. Then you use the tactic of boatbombs - you put a weak army as a bait, the main forces are waiting in the sea. Then when opponent attack, you drop the armies and smash him. Morale penalties do not affect efficiency, so you crush it's armies bu small pieces. This tactic, when used wisely and with good retinues allows to win war even when the opponent have more troops, but they are spread across a country, like HRE or Byzantium.