Hello, is there a practical way for a do it yourselfer to test a fiberglass bass boat (2007 Stratos) to see if the foam is waterlogged?

Very simple.
Plastic handled screw driver, tap the handle around the motor mounts, thru hull fittings, and around the stringers. It should give you a nice clean tap sound, if you hear thud, that's water.

One way to check is comparing pdf or spec. sheets on boat and motor for weight. Trailer may have the information on the serial / vin tag. Add for fuel, batteries, etc. Take to a public scale, like Loves or places that weigh for scrape iron. I just paid 10.00 locally for a trailer weight for homemade trailer registration. If there is a considerable amount of extra weight, there is a problem. Some insurance companies have a meter to check moisture content, especially in housing adjustment / claims, but I would suspect some moisture in most older boats. Hope this helps, Steve