I've had two staffing agencies call me. How do I know which ones are legitimate and which ones are scams?

I've been throwing my resume out online again. I've had two agencies call me and ask if I had time to talk. They had my resumes from an attempt to find work three years ago. I told them a bit about what I'm hoping in a job and what my strengths are. They both said I can come interview and they'll let me know if their clients have openings.

So my questions, How do I know if it's an agency worth giving my time to? Is there a database online that rates them?

How do I know if the whole thing is a scam (none of them wanted me to sell insurance, I get plenty of those emails)?

Is it worthwhile to use this method of finding a job at all?

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