If the TV wasn't in the rain, I take it home (like I just did tonight on a Sony flat screen), and plug it in. Sometimes, the TV will not have a cracked screen, so I can see it power on and then see the mother board is fine. Sometimes I'll plug a video game system in to test it that way. The only other things I check closely are the ports to see if there's any damage to them. If the screen is cracked, I can still get the same testing in the same fashion, minus seeing a clear image.

You just know when it's good. If I'm ever not sure, I'll sell the main board for parts if it's worth the effort to extract it. Out of every board I've ever sold, I only missed one issue, which was an ethernet port being busted, which taught me that lesson. He kept it for a partial refund. When there's a cracked screen, you get a gut feel that's why they tossed it.

I just scored a massive haul in just my first 2 blocks. Easily $900 or so from three main finds. Going back out now to do the rest of the run.