how to know solar panel watts

Unless the unmarked panel is "about free"--I would suggest avoiding it.

You can measure the Voc and Isc (voltage open circuit and current short circuit in full noon-time sun) to estimate the panel's ratings... You can also count the number of cells (each cell in series is Vmp~0.5 volts--36 cells = ~18 Volts Vmp).

A 100 watt solar panel is ~$100 to $150 new (can find for less or more).

If you purely want to play and can get it for $20 and you can test Voc/Isc -- Perhaps it would be worth it (assuming the voltage and current ratings are appropriate for your needs).

In the end, you should first define the loads you want to support--Then start picking the hardware. Buying hardware (and especially unmarked hardware) for more then pennies on the dollar is usually going to be waste of money. But if this is for pure playing around and you have the extra money--Not going to hurt anything.

-Bill