I would bring a bright flashlight and a pointy knife and crawl into the boat stern. Clear away whatever it is so you can see the inside of the transom. See if you can stick the knife into the wood with ease which is bad or if it is hard which is good and look at the wood to see if there is any visible rot.
As for the outside,try and move the engine back and forth and sideways. If you feel a lot of give,you have a rotten transom.
The 1985 starcraft Islanders are 19 footers, the 18 footers are Holidays. If this one has an outboard it probably is a Holiday