That's a good question, it's tough. There are definitely fake jobs out there. Some lazy recruiters will cut/paste real job descriptions straight from the client, but change the city to one nearby to throw off competition or applicants. Others will have fake jobs to collect resumes.
I started doing recruiting last year (my background is dev/mgmt), and in my ads I make it clear who I am with a link to my company site and LinkedIn profile. I say what groups I run, so they can verify on Meetup if they wanted, as well.
When I get your application, I'll email you requesting a phone call (or a cup of coffee if you prefer), so your investment is very little and you can always hang up if it's not for you.
On a phone call, I'll tell you exactly who the company is, because I need to know if you applied before. Most candidates do a quick email search if they can't remember. Submitting you when you've already applied, and maybe even interviewed, makes me look bad and you look worse, and I don't want either. If the recruiter won't tell you the client, just walk away.
If I reach out to you directly, I'll tell you up front who the client is and give details about the position.
A number of devs come to me by referral because I've been active in the local dev community for 15 years. I hit a lot of meetups and code camps. If you ask friends, they may have a recruiter they recommend.