Ian is correct about dismantling if you really want to completely understand the level of corrosion. But you can get an indication by removing the thermostats and observing the amount of corrosion around the base. Get one of those camera probes and look down in there as far as you can. If you want to go a step further, remove the exhaust manifolds and see what condition they are in and use the camera probe to look inside the exhaust ports.
Two other major things you should look at - 225's in with SN's of BAGJ 1500324 and BAHJ 1500022 and lower had the exhaust manifolds that allowed water to get too far up into the engine and blow the HO2 sensor. In the worst case, it would cause a hydro-lock and destroy the engine. You need to find out if those manifolds were replaced with the re-designed ones. A lot were under warranty. You can call Honda Customer Service Customer Service at 770-497-6400. Give then the SN's and they will let you know if the manifolds were changed out under warranty.
Also, some of the 225's in approximately the same SN range had a problem with the mid-shaft (drive shaft) bushing. It would fail due to a lack of lubricating water, start to wobble, and could eat a hole through the lower cowling. You need to drop the lower unit to check it. Fairly easy to replace.
After running the engines, pull the oil dip stick and smell for any oder of gasoline and look for any indications of water in the oil.
Pull the magnetic fill bolt on the lower unit and check for any metal filings larger than talcum powder.
Do a thorough compression test. Compression should be over 190 psi across the board with no more than a 10% difference between highest and lowest cylinders..
