I assume when you say "fake" you mean they are consulting firms that are making the posts. Here's what I found out about weeding these consulting firms out:
- If immediately after applying (within 5 minutes) you get a phone call, then you know it's fake
- You see the same ad frequently posted or the same post is bumped daily, then you know it is fake (this might also be an indicator of a real company, but with high turn over rates).
- You notice a company has the word "Staffing" or "Consultant" somewhere in it, but no actual company.
- After "applying" to the job, you get requests to move 1000s of miles away from your current application area.
Eventually you'll be a master at spotting "fake" or spam advertisements. I personally think they should make a law on posting these sort of ads on job postings sites, but my guess is they are their #1 customers compared to a firm only posting one job.
But as always try to go to good source for jobs. Don't go on something like Craigslist, LinkedIn (at least random communications you get on a public profile), or ads on your local newspaper. If you do use those sources, be skeptical of the post and pay attention to small clues. Also try to figure out how companies are able to post on the site. Is anyone able to post? Or is it vetted prior to posting?