I can't speak for the Marines, but I've been through a ton of Army investigations of shooting incidents. The National Guard tends to be somewhat trigger-happy and there have been some screwed-up Regular units including one particular brigade of an otherwise-elite division to which I once belonged, but with very few exceptions when things are breaking fast and innocents end up getting shot in ambiguous circumstances where the troops had to make snap life-or-death decisions that proved wrong afterwards, everyone is bummed out about it and the Soldiers generally try to render aid to the wounded instead of screwing around with drop guns and other hi-jinx.
Unfortunately, the main distinguishing characteristics of an insurgent ambush party are sudden arrivals and departures, cell phones (both for commo and in varying levels of of sophistication as triggering devices) and camcorders so they can put their handiwork up on Islamist websites, and there may or may not be visible small arms in sight. This makes them virtually indistinguishable from journalistic stingers (who may be at the attack site due to sub rosa connections of their own with the insurgents anyway), especially if said stringers are paying for an armed bodyguard to run around with them. There is kind of a Darwin thing going on with the journalists, which is nobody's fault but their own.