Most speakers are mounted to the front of the box and the back is not removable. You can remove the woofer. It might be marked with a make and model. The recommended power for the woofer would give you a pretty good idea of the wattage that the entire speaker can handle since the tweeter usually only uses 10-20 watts more than the woofer.
If you want to sell them it doesn't matter since that don't have a make or model therefore little to no value. Sounds like they may have come for an all in one system and your grandpa replaced the original electronics with the Yamaha receiver when it broke.