Green Energy

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.

The solar panel technology has evolved in leaps and bounds in the past 20 years. Current panels are more efficient and resilient than old ones. I'd recommend saving for a new installation if you can.

In some places you can get a loan against your house equity for things like a solar panel installation if you install it on the roof. The thinking there bejng that the value of the house increases as a result. There may also be subsidies you can access.

That being said, you can test an used solar panel with a decent multimeter, by measuring the voltage (DC volts), short circuit current (DC ampers) and operating current if you can (this one is easier with a clamp meter, but you have to measure it on a live installation). As long as the values are reasonably close to the panel spec, you should be fine. There are tutorials out there on how to actually do it without blowing the fuse on your tools.

Depends on what you want to do with them. Some simple project that's easy to work on? Sure, worth a try. But if you plan to do a complex roof installation for example, the cost of new PV panels is really the least of your concern.

The absolute cheapest 420Wp panels are €50-80 new. That would be a roughly 1.7m X 1.1m panel. So 35€ for the risk of buying scrap and them having been degraded would be a bad deal. If you can get the rest of the needed installation parts with it, it might be worth it or if those are new panels.

If you want to get started there are solar home kits online, which are rather easy to install, if you have some very basic diy skills. The electronics is basically plugin in some cables and the difficult part would be actually setting up the panels. On a flat roof that might just be screwing on some metal stands and drilling a hole to run the cables through.

Solar is only getting cheaper and more efficient. I would buy new.

Just informing you: You can see the power output of PV panels by looking at the Watt number. It says something like 400 Wp (Watt peak - i.e. Watt under direct sunlight). Voltage is more or less irrelevant.