When a character sets an ambush, he/she makes a dice roll using his/her Tactics (including setting ambush specialism) + IQ.
The dice roll result shows how good the ambush situation is (this determines how difficult it is for people to spot the ambush, see below).
There is a modifier on the dice roll depending on how good the terrain is for an ambush, from -3 for flat open country, +0 for hilly country, +2 for dense forest, +4 for underground or in buildings.
When a character walks into an ambush situation, he/she can spot it and get advance warning if he/she makes a dice roll using his/her Tactics (including spotting ambush specialism) + PE, and obtains a dice roll result that is greater or equal to ambusher's dice roll result (see above).
Poor concealment of any ambushers could also give away the ambush.