How can I find out if a recruiting agency is legit?

Factual Questions
1

I’ve posted a profile and resume/CV on various sites like ZipRecruiter; today I got phone calls and emails from two recruiting/staffing agencies who apparently saw a job opening that I might be a good fit for. One was Sunrise Systems; I’ve seen that company name pop up several times in job alerts, but then read negative reviews of the company, some outright calling it an identify theft scam. There are also glowing reviews of the company online. This being the internet, I thought of two paranoid scenarios: first, the negative reviews were perhaps from disgruntled former employees or rival companies trying to sabotage a legit company, and then that the glowing reviews might be generated by socks or something to boost an image of the company as reputable. Anyone can make up a review blurb, attribute it to a random name, and how can you check? When I told Sunrise Systems that I wanted to do more research because I’m being cautious of potential scams, he hung on me without another word. Seems like a red flag, wouldn’t you say?

The other company that contacted me was Revolution Technologies. Haven’t found as much about them yet, but still digging.

So my questions:
how do I weed out the potential scammers and fly-by-night recruiters from the legitimate, reputable ones?

Do large corporations often consider only applicants represented by a staffing or recruitment agency?

2

Yep, rather a big one, to my way of thinking.

3

No. Recruiting directly, costs less, as they avoid the agency fees.

4

I’m retired now, so this is my impression from a few years ago.

But I concluded that recruiting agencies were ALL scams.
They offered the same job openings that I saw offered directly by the hiring companies – they just wanted to charge fees for connecting us. But I could do the same by sending my resume directly to the hiring company. I never found any ‘un-advertised’ jobs that only the recruiting agency knew about.