Have a plan and research goals plotted beforehand, use the mod inspector to help. Know your nations strengths, but try to be adaptive to what you are facing on the field... I know, non-help there.
Try not to engage a large hostile player army if you don't have any idea of what it is capable of. But them attacking a 1 PD province would give you that intel.
With a bit (ok, a lot of) familiarity with the spell list, you could figure out their rough research levels from the spells they cast, and checking their mages for gems would give you a rough idea of what 'big spells' they're bringing to the party.
If it's a game where the score graphs are on (or you have a site that reveals the research graph) you could get a vague guesstimate from their research score, though.
Generally, I'd say it's a question of: What does he have; what do I have to bring to the table, what can I research in the next turn or 3 that would have a significant impact, do I have enough ablative territory to accept territory losses to allow me to blob/tech up just a bit more, or am I just able to smash it without expending too much effort?
But that's just me, everyone has a different style/idea of what is fun in this game, I enjoy having to sit back and do extensive sifting through the spell list to sort all of those things out.