Hey I recently took the diff off my 240sx and I was sure it was open diff because it would only spin one tire. I took it off and when I spin the shaft the driveshaft spins, both sides spin instead of one and it has a 4 bolt back cover. What can I look for in the internals of the diff that would be different?

Put car in gear or park and then with both wheels up in the air spin one of them by hand, if the other goes in the opposite direction it is a single trac open differential.

Assuming the rear cover hasn't been changed, the easiest way is to look for an orange sticker that will say LSD. AMC is right about the wheels turning in opposite directions, unless the diff is a VLSD. If that's the case then with the oil cold it will look like an open diff using that method. Your best bet would be to pop off the rear cover and post a photo up. It would be easier than trying to explain the differences between types of diffs.

An open single trac has virtually no resistance at all, I would assume that a viscous drive would be like a silicone fan clutch, there would be at least some resistance.

You're right, there would be a bit more resistence than an open diff. However, for individuals that don't have a good sense of drag, the difference to them could/would be negligible. Since I'm assuming this is the first diff that the OP has messed with, I doubt that they would know the difference in resistance between the two diffs.