Are you referring to actual phone calls? I mean everyone should know by now that phone numbers can be easily spoofed, we see that in every call claiming we need a new credit card or car insurance. The easiest way to see if a call is a scam is to force them to go off-script. Like when they start asking for personal info like your SSN... you called me, why don't YOU know my information? Of course they'll say they need it to verify who I am, and I'll just tell them that they should already know who I am since they called me. Another big tell is if they want more than just the last four of your SSN... absolutely no legitimate agency will ask for the entire thing over the phone.

I guess it just depends on how much free time you have, but sometimes I just like messing with these people to waste their time. Some will get downright angry when they realize you have no intention of falling for their scam, but mostly they just hang up.