Look and check their hose management, and wire management. You can know if they didnt take the time to route hoses or wires neatly, they definitely skipped steps elsewhere. Overly long heater hoses or rad hoses that are all over the place just scream rushed job to me. You could look for kinked hoses or wires that arent properly sheathed. There shouldnt be any straight up exposed wires in the engine bay, they should all have some kind of abrasive protection on them.
Personally, if i see any of those nylon insulated wire connectors, butt crimps, or terminals, I would turn around and walk away. For Engine wiring at least. A light bar or something like that is a different story, but for important stuff, those crumps are a no go. If you want to nitpick even more, electrical tape instead of heat shrink is a big one for me too. If you can get up close to it, check out the motor mounts. The welds on them, and if they are poly mounts or oem style rubber. The rubber ones are preferred. With V8 swaps there are swaps that are done just to get the motor in there and to work ok, and there are some that are done with detail and care for long lasting reliability