I've been a tower controller for a few years and having thought about it a bit through training others, realize I have a general sort of filtering system for aircraft ID. (Often you don't realise how you do something, until you have to explain it to someone else.You just do it. This action -teaching/explaining-can work to improve - or sometimes invalidate - your own model.)
Find photos of all the aircraft you need to be able to ID. Pref including some different angles.
Obvious things to compare: No. and layout of engines, High/low bypass. Wings high/low, swept/not. Wheels, (Generally maingear) nr. of axles, nr of maingear legs. Tail, T, cruciform, regular. Dihedral?
Cabin- window shape, position (high or mid) no. of overwing (and other) emergency exits. (Eg check out the window shape on the Fellowship compared to the DC9 series) Cockpit windows, general layout, angle windscreen blends into the nose, size.
Where fitted, Airbus winglets look distinctively different from Boeing winglets.
Just glance at the whole aircraft to categorize it (eg that's a low wing narrowbody twin jet, underwing engines, or, that's a low wing jet with two underwing and one tail engine) and then go through the above list - as few or as many items as req to make a positive ID. In the above bracketed examples, a low wing narrowbody twinjet definition already eliminates many types and then draws you to the obvious "big 2" family, Boeing 73#/AirbusA 318/19/20/21. The second example makes it either a Tristar, or a DC10/MD11. Then look to the differences that you have learnt differentiate each aircraft in a similar group to ID it.
Lastly, although the post is a bit long, don't sweat it too much. If they want you, you're not going to be shown the door 'coz you couldn't tell the diff between an F28 and a DC9.