A mechanic told me this trick. Get an old radiator hose (I used a rolled up towel). Start your car. Let it warm up. At idle, stick the hose gently into the fan. If it stops easily, it's bad. BE CAREFUL if you ty this.
Mine stopped VERY EASILY & took about 30 seconds to come back up to speed. I put a new one on (4 bolts) and my problem is solved (temp was creeping up when I stopped at 3 or 4 red lights in a row).