But if you lick it, you claim it and have to buy it. It's yours after that, just like your food. You lick it, it's yours
Seriously though, check everything that could corrode or rust, especially cast stuff like the manifolds and electrical connections. Those are the most obvious culprits inside the boat. The trailer is a dead give away though unless it's galvenized, which also points to salt water use.
Ask them about it and if they did, how they flushed it and cleaned it. Most people are honest enough when asked directly. If they dance around the question or give you some vagure reply or some bs you know isn't true then avoid it.