Look at the rear mudgard (fender in US terms).
If it curves down slightly then flicks up at the back, then it's a Custom
If it curves all the way down, then it's a Classic.
Look at the rear indicators (turn signals).
If they're mounted directly either side of the mudguard, it's a Custom.
If they're on a seperate bar, near the rear brake light, it's a Classic.
If the mudguard is plastic, it's a Custom.
If it's metal, it's a Classic.
An easy way is a Google Image search for each.
Side by side, the differences are stark!!
Forks are different sizes, mudgards are shaped differently, indicator placement is different, as are the headlights.
In general, Classics look bigger, have different parts (seats, bars etc) and have more metal bits.