Look for "RoHS"[1] on labels and electronics. That governs a list of materials, but I suspect the most common is lead. Among other things, this is the origin of significant usage of lead-free solder, which has led to its own set of issues in electronics (brittle joins, tin whiskers, etc).
Lead-free solder has since improved greatly. These days it's pretty hard to discern from leaded tin other than the fact that joints tend to not be glossy.
Especially if you're a hobbyist: Go for lead free solder. it's no worse to work with than leaded but a whole lot less toxic. (Still don't breathe the fumes though, solder flux is nasty stuff)
Seems there was a lot of hoopla about lead-free solders back in the day, but it seems industry did figure out such solders and protocols to use them properly, and contrary to various predictions, the sky hasn't fallen.
Looking at the state of e-waste "recycling" in 3rd world countries, I personally think it was darn good we got rid of lead solder.
[1] https://en.m.wikipedia.org/wiki/Restriction_of_Hazardous_Sub...