Any individual device is pretty simple...power in, ground out, maybe an external sensor via twisted pair or something. If you've been careful, those should all check out okay.
There are some boxes that need more complicated post-installation checks, and these are usually in the installation manual (I have a GNS-430W, which has a whole checklist for post-install check-out). Go through each of those, if you have any.
Trickiest part, from what I've seen with most builders I know, is interconnections between devices. A Garmin talking via an ARINC box to a Skyview, e.g., or a certified WAAS navigator feeding data to a different brand's ADS-B compliant XPDR. Make sure everything is talking to whatever it's supposed to using the right protocols, data speeds, etc., and that any data is indeed being sent and received properly. Exercise those functions that rely on that communications and make sure they're working properly.
First flight you're really only going to care about watching the engine gauges for any issues, and keeping a dirt simple flight profile, but you don't want a bunch of s**t alarming for no good reason on that flight, to distract you while you figure out if it's real, if it's important, and how to shut it up. Get it all working properly on the ground beforehand. (ETA: Oh, and that means, also, checking out all of the configuration stuff for the Skyview, most importantly including alarm setpoints. Double-check them all.).