That to me is one of the hardest things. There's a lot of shady people out there sending bot traffic and doing other unethical things for a quick buck in the solo ad business.
Three important lessons I've learned:
1-Never buy from a vendor until you have spoken with them in person, either over the phone or on skype. If they do not want to speak in person to answer a few questions, they are either hiding something or at the very least are not treating their solo ad business like a real business. If they won't treat it like a real business, they don't get my business.
2-Ask if they would like to see your ad copy. I've found many say things like "I know what my list responds to, so I'll just write the copy myself." If they are not even interested in SEEING your copy, that's a red flag that they might be sending you junk traffic where the copy doesn't matter.
3-Ask them point-blank if it is going to be real traffic or bot traffic. If they are using bot traffic, of course they are going to lie to you here. Keep your ears raised for any fumbling of words or other signs that they might be lying, or hiding something. Tell them you’ve had experience with bot traffic before and have learned how to spot it. Trust your gut.
Anything you wish to contribute to this list?