If it has a model number on it, you can probably find out when it was first released to get a maximum age.

PV panels can have defects that are invisible to the naked eye tho so its not easy to check without actually testing it out. Afaik measuring the voltage without lab conditions would tell you if it were completely dead but not how healthy it is.

For 35€ i would personally just buy them, but you didnt write anything about the inverter to turn the DC into usable AC… Is there one that comes with the panels? If not you would have to buy one and that can range from like 100€ to much more depending on the total power output / wattage the panels have.