It takes awhile to learn how to spot them reliably, but here's a very quick guide:
Shortwings:
680T and V: no eyebrow windows, nose lights.
680W: no eyebrow windows, no nose lights
680W and 681: Eyebrow windows, no nose light.
The difference between the short wings and the longwings are at first hard to spot, but becomes very easy after awhile. Two dead giveaways are - the longwings have a little portion of the wing that's straight between the fuselage and the outer dihedral wing, inboard of nacelle when viewed from front. Also, the props are bigger and further away from the fuselage.
Longwings:
690: Rounded dorsal fillet similar to shortwing models, but with the big props, big wing.
690A/B: straight dorsal fillet with a little air scoop at front.
840/980: no dorsal fillet
900/1000: dorsal fillet, airline style windows and no picture window.