How do you know if your lower unit is bad? part 2?

Ok so i've asked this question in a early thread, but i didn't provide enough information for others to understand what i mean by lower unit being bad. I took my outboard/boat to a mariner repair place, he told me that my lower unit is bad because it leaking oil from the vent where water would go inside the motor to cool it. Also i went to another place and they told me to put it in gear and try to turn the propeller to see if oil leaks out and it did. But how do i know if it actually bad. the motor is a mercury 65hp 4 stroke unknown year but the casting seem pretty old enough.