How do I know of wetness on the inside of a silnylon rain-fly is condensation or rain that has come thru?

The inside of my new TarpTent Scarp1 fly was wet after a night of light rain.

I have been told by other backpackers that particularly heavy rain penetrated his silnylon TarpTent and he solved it by getting it recoated. But if this is condensation, recoating it would not help me.

A light rain wouldn't have exerted sufficient pressure to push through sil. I vote condensation.

My rule of thumb is that if the moisture seems to cover everything uniformly, it's condensation. If it comes from one spot or along a seam, it might be a leak.

You can get quite a bit of condensation in single-wall tents. Sometimes rain or wind will knock a few droplets of it into your face. Not enough to really complain about, though. It just means the tent needs to air out at the end of a trip.

Testing at home would be a good option. Hit it with your garden hose. (Not full blast of course unless your testing for hurricane conditions)

Condensation is formed from humidity (natural in the air and wet gear) and your breathing while sleeping in the tent. You can pick your test conditions and minimize the effects of ether during the test. For example set it up over your driveway instead of the grass. Good hot weather, with low humidity. Avoid cold water in the test. etc.

Hope that helps. Good luck.