The way you can identify the jobs that are going to H1B visa holders is the way the jobs are advertised. The federal law requires that you post the hours of employment in the ad. So what you see typically is a job description which is essentially so specific it is a fingerprint of Hadji - the H1B holder they really want to hire, then it lists a salary (let's say $90,000/year) and then hours M-F 9-5. That's the giveaway that it is an ad merely meeting the requirement that the employer advertise for some period and then fail to come up with a U.S. citizen meeting the job requirements. Another giveaway - it's in the smallest type font available in whatever publication it is located.
Here's another trick I came across about 12 years ago: Running the job ad in a publication unlikely to be read by the engineer or scientist seeking a situation. A colleague brought to my attention a teeny ad that appeared in the Wall Street Journal, for a C programmer with a PhD in mathematics in fractal analysis. Well, I program in C and I have a PhD in mathematics in harmonic analysis (pretty close to fractal analysis). The ad said $54,000, M-F 9-5 and gave the address of the Georgia employment agency. I wondered why the ad didn't say what company the position was for, but I had a pretty good idea: Iterated Systems (if I remember the name correctly), a company using fractal analysis to compress images. (Microsoft used their technology in their Encarta encyclopedia.) So I placed a telephone call to the company to ask about the position. The woman who answered sounded positively angry that I would call. It all suddenly clicked together: they had someone in mind (probably a graduate student of the professor that started the company), a foreign national, and they had to play this game to "prove" no Americans were available who could do this job.
What was infuriating about this was that at the time, the unemployment rate for US citizens with new PhDs in math was 14% (this was 1992), with maybe 10% underemployment on top of that. This company would have had no trouble at all getting a US citizen for that position -- if they were sincere.