Peter Johnson @Alcebaid was very helpful, gave me some tips and then made this guide which I edited a little bit for even easier reading.
There are four main battle tanks: T-64, T-72, T-80 and T-90.
T-80 is hardly seen.
T-64 has considerably smaller wheels:
T-64 had track segments made of two parts:
T-72Б had 1-part-segments:
Starting from T-72БA and up it was 2-part-segments again:
T-64 exhaust is at the back of the tank (sometimes they attach a log there). T-72 has its exhaust at the left side.
T-72Б3 (shape of exhaust depends on engine):
Older T-64 had an infrared light on the left side of the turret. T-72 on the right side.
T-64 has one long box for spare parts and instruments. T-72 had three, left, right and behind.
T-64БB (the pipe is for underwater driving):
Backbone of Ukrainian army. IR lamp on left side. "Kontakt-1" ERA (reactive armor)
Deep T-64 upgrade. IR lamp removed. "Nizh" (knife) ERA. Rubber sheets attached to turret.
Since september 2014. "Kontakt"-like ERA. Bulat-like signs.
Old tracks, "Kontakt-1" ERA, IR-lamp on right.
T-72БANew tracks, meteo-sensor, "Kontakt-5" ERA (although sometimes older)
IR-lamp removed, new Sosna-U sights (with IR-possibility)
Deep upgrade of the T-72Б. "Shtora-1 lamps on both sides.