Is there a way to tell if your opponent is playing zone or man-to-man D?

[Steppenwolf]

Any tell-tale signs?

Well, its kinda connected to the way your opponent is playing, offensively or defensively... If he is attacking, it's likely that he is using offside traps, and if he's using offside traps, it's likely that he is playing zonal defense... but, if he is playing defensively using counter attacks, he might use man-to-man defense (not necessarily)...