mercruiser small block, how can I tell which motor is in the boat, 305 or 350

Is there a serial number or horsepower rating on it anywhere?

Look for the casting numbers on the block.

most will have a fairly large 5.0 or 5.7 outcast on the block

usually looking down at the back of the block, I dont see it

If you can get the casting number of the head that might lead you in the right direction also. The 305s I have worked on had a smaller chamber heads. Several people used the 305 heads on stock car engines to raise the compression.

try looking on the block area on top at bellhousing area behind dist.