You'll often see a female that isn't in heat (bred or open) make a pass at another cow that isn't in heat, and if she does it one time and then goes on about her business, I wouldn't worry about it much. Seeing one do that DOES make you wonder, and I would watch them closely for the next 12 hours to see if you see more activity. If she continues to try to mount other cows and seems alert or restless, my guess would be she's probably cycling. We sometimes have issues with a cow being in heat and none of the other cows paying any attention to her, especially after they've been recently bred. If a female is cystic, she may cycle every 3-10 days...breed her after standing heat just like you normally would and give her a shot of Cystorellin at the same time you breed her. That should take care of that.
Standing heat is by far the best sign that one is in heat. If you can't catch her standing, you just have to go with your intuition and experience. If she's chasing other cows around and none will pay her any attention, that's a pretty obvious sign, too. One attempt to mount another cow that doesn't stand wouldn't concern me much unless I knew it was time for her normal cycle and was having trouble detecting heat...then I might think more of it.