So I found a shop that isn't an hour away to play at, and I got my first game in over 2 years in the other day. However I ran into an un-expected situation mid way through the game. I suppose I should start at the begining.
We were doing a 2 on 1 Apoc battle at 3,000 points, I was the one and I was running my CSMs. I had to proxy a couple things cause I grabbed a wrong case. No big deal, right? Just make sure to tell your opponent what you're proxying and you should be fine. I do have to admit it was funny as anything to proxy my Biker champion with a model that has a heavy flamer. Anyway, I went through all the models that didn't have the "standard bits" and made sure my opponents new that this Aspiring champion with the demon claw had a power weapon, the raptor chapion with the Whip-lash chain weapons counted has having Lightning claws, the Tech-marine looking model was an Obliterator... all the way down the line. Nothing much seemed to come of it until the begining of their turn 3.
My Biker squad came in from reserves and I used my Strategic asset to come in on a board edge that was close to the rear armor of a troublesome Baneblade, intending to Melta-gun/bomb the snot out of the blasted thing over the next 2 turns. Flanking the BaneBlade was a Land Raider Crusader. With the exception of the Assault Cannons, I didn't have much to fear to my bikers in reataliation since they're running an Icon of Nurgle making them Toughness 6 and I was out of range of the Multi-melta on it. So I didn't really worry about right away, my priority was to take out the BaneBlade. so I managed to land 3 good shots with the pair of Melta guns and the Combi-melta and managed to take down a structure point and render it unable to move next turn. I then charged in but failed to do any measurable damage (damn melta bomb was a dud lousy good for nothing 5 points...). So the turn ended. I knew the Space Marine player would retaliat against my offending Bike squad cause... lets face it. 10 Toughness 6, relentless marines with twin-linked bolters and a pair of Melta guns strutting around your lines isn't something you want to ignore.
So it went down like this: He turns his land Raider crusader to face my bike squad and then says "Okay 2 lascannon shots at your bikers." Turns out he had been running the crusader as a Standard Land Raider the entire time, but had neglected to tell me so. Had I known I'd be facing down a pair of T.linked Lascannons, I would have altered my plans.
So here's where my question: before you actually start putting models on the board, how extensively do you go over your army with your opponent? Is it driven more as a point of courtesy or a point of good sports man ship? Do you have a lot of conversions in your army that may look confusing as to what they may represent in game? If so, how conscious are you of telling your opponent about them before you start the game?
foot note: Tzeentch answered my prayers when the Space Marine player went to Dis-embark his 10 man terminator Squad from the Land Raider and I informed him that a Land Raider can't carry 10 terminators, so we agreed that he lost 5 of them.