[tsubaki]

And I honestly don't know that without prior knowledge you can recognise a bad transom unless it's severe.
Mine barely flexed with me standing on the motor with it tilted up and jumping like a horney monkey, it wasen't till I was going to do away with the cap that I discovered how bad it was.
All in all the only place that had good wood was directly in the center where the motor bolted. All the wood on either side was like dust.
If the motor bounce test don't prove anything, remove the cap and probe down and to the sides with a long skinney screwdriver, antenna or coat hanger, this may give some indication.
Don't be shy about drilling holes in suspect locations, these can be patched with Marine-Tex if a pour is not needed at this time.