I have to disagree with Rainboe, with a come-back like that, roundthebend will do fine in ATC.
To try and help out, I've no idea how this is tested in stage 1 selection (maybe there's more info somewhere on Pprune) but here are a few hints......but I should warn you that I've been an aerodrome person for the best part of 30 years and I have trouble with distinguishing between many types.
A300s always struck me as looking different to everything else, they seem to stand higher, taller fin and big engines. A310s are short 300s. The 320 series are tricky, 318s are noticeably short, 321s are noticeably a little bit long, but 319s and 320s cause me difficulty unless they're next to each other!
Boeings - I guess 73s and 74s are easy and there's nothing long and thin like a 75 in the air. The 76 and 777 are fatter.
When they're high up I find the wing shape on the Boeings and Airbuses are completely different - can't describe it but see if you can find pics and you might see what I mean.
I agree with everything that's been said about the MD-80 series. I gather that the end of the fuselage under the fin gives a hint to the model but it's not foolproof (this courtesy of a spotter-ATSA colleague) - some are round and others look like they shut the hangar door before the thing was out.
With the smaller types I'm afraid I'm not that familiar - I just say "follow the Fokker" or whatever! But I think it basically comes down to length.....there's a joke there somewhere but I won't bother.
Hope this helps and good luck.