only by watching the replays. 

They probably are using Sigils + Blood of the Fallen + lots of health armor + Journeyman's Treads

If you tell us who the specific AI are, we could probably tell you.  Frankly, I'd just recommend playing a game vs the ai (modded) and then check the replay.  They buy good items. 

Thx - we did a good job picking up the items for the ai.  So - if you check a replay vs the modded ai, you'll see what most solid players would recommend for ability and item decisions per level. 

If you do get sick of playing against the ai, do a search for enfos mod - that's fun in SP and MP and changes things up alot.