If it's something that is plausibly applicable to me (Yes, I was a customer of that company during that time, etc.), I just go and independently research it real quick, and if it's legit I go sign up. I have gotten a few payout checks from some, usually years later, and probably for like $5. It's pretty easy to look up the information, because there's generally news about it and it will say the law firm handling the suit and provide a portal link where you can go to confirm you're part of the class and you can just skip whatever email you got, if you feel nervous about it.
The biggest one I've personally been involved with is the massive KIA class action lawsuit and that is something that personally cost me thousands of dollars out of pocket, and that one I got an actual letter from KIA in the mail informing me I was a member of the class, who the law firm handling it was, where to sign up for updates and to provide specific information if it affected me financially, because they have the VIN for all the vehicles affected and my car is registered to my name and address under that VIN.