I'd recommend not taking a chance you'll miss something and take it by your local bike shop. They have a tool that can correctly re-align a bent hanger...this is much more likely then the derailleur being bent. Also, the mechanics have looked at tons of bikes up close and will notice little things that you might miss. Many LBSs will do a post crash check for free (the one I hang out at will).
That said, here's what I check when someone brings me a crash bike.
Good look over of the bike in it's current state before I begin. I'm looking for signs of how the bike hit, scratches and the like. If I wasn't the rider, ask the rider how the accident happened. This will help me to focus more intently on some areas. Don't let this make you focus to the detriment of the rest of the bike. Before doing anything to a bike that may have legal/insurance claims make sure you document with photos...if appropriate get clearance from police/lawyer to start having the bike put back together.
Frame soundness. With the wheels off, carefully go over the frame and fork for any cracks, dents, bends, or scratches. Scratches are okay, but they can help pinpoint impact areas. Wavy paint can show where a frame is subtly bent. A tiny crack on the surface may be deeper/bigger under the finish. Most of the time we think of cracks in conjunction with carbon, but a aluminum (or even CroMo) frame can get stress cracks as well due to a collision. With metal bikes, bends and dents can weaken the integrity of the frame as much as a crack does a carbon bike. The bigger the dent or the closer it is to a weld, the worse.
On metal bikes, look at each weld carefully. A bad weld can hold for years and then separate due to stress. I haven't seen this on a bike frame, but I've seen it on other equipment.
Steer tube/headset - is it still straight. Often the handlebars will be pushed off straight with the tire. This can be fixed quickly (loosen stem, realign, tighten stem), but is another indicator of stress. If not straight, spend some extra time here. Either way, check that everything moves as smoothly as it did before. The angle/alignment/tolerances within the headset are small and a little bit of being off can cause binding. Loosening the compression nut (top of steertube) and re-tightening can often realign. If you haven't worked on a headset before, get a hand from your LBS to show you torque and order.
Brakes - quick and easy, are they damaged? Do they still work? Scratches are okay as long as the scratch isn't rubbing anything.
Drive train - the hanger (as yours probably is) is often the first casualty. Remove rear derailleur and check (LBS has the special tool). Next look to see if there's any damage (other then cosmetic scratches) on the RD, especially where the shifting cable is routed and tightened. Move up to the front derailleur. This can be bent pretty easily. Usually it can be bent back, be careful though...easiest to have a bike mechanic bend it back into shape if needed. Check the chainrings for missing teeth (note that on some cranks, there are supposed to be missing teeth to aid in shifting) or bends. Check that the bottom bracket is still tight and that the cranks rotate smoothly with no binding. Check the chain, ideally remove it and let it hang - that's the easiest way to see single link bends, it should hang straight...if you leave it on the bike, slowly rotate the cranks while watching the middle of the chain from above, you are looking for any bend - it will present as an angle between two links.
Seatpost and saddle. Saddle may have been turned, but even if not, I recommend completely removing the seatpost (mark the height for reassembly) so you can look for bends or cracks. If bent it won't come out/go in smoothly. If cracked, you need to get a new one and do a careful inspection of the seat tube.
Wheels - check for cracks in the rim, cracked nipples, bent or loose spokes. Take off the rubber and put on a truing stand checking for true, dish, and roundness. Check the rubber for skid wear and/or gashes.
Rider - keep aware of yourself for the next several days. You can miss that you also hurt something else because it doesn't start really bothering you for a bit. My last crash, my shoulder felt fine for a day then started bothering me...it was minor, but if I wasn't paying attention I wouldn't have connected where/how the injury occurred which can be important to the doc if you end up needing medical intervention.
