I think my bitch is coming into season boys starting to grit there teeth and trying to mount her just wondering if its worth getting her blood tested and how reliable are they

You don't do a blood test to see if a bitch is coming into season but rather to see if she has ovulated.

The easiest way to see if she is coming in is to do the tissue test every morning. Dab her nether regions when she first gets up. If there is blood then she is in season. Though not all bitches will actually have a show of blood. There is such a thing as a silent season, where it can be difficult to tell :-(

If you suspect that she is in season then you really should separate her from your males. Preferrably nowhere in the household ;-) They will possibly start fighting for the breeding rights which will cause you stress and possibly cost a lot of money at the vets. It would be best if you could put your bitch in kennels, or a relative/friend that you trust not to let her out near other males.