
Blood tests that measure the serum progesterone levels are the most accurate and what you would use if doing AI, and are done at a Lab, so you have to wait to get results.
You can have Premate or other tests that are done in house and the results checked against a colour chart (either varying shades of blue or red) when the right shade comes back from the test the bitch is ovulating. These are less accurate but close enough for natural mating.
The swabs a re basically a smear test. The predominating cells in the vagina change during the season and a knowledgeable vet can estimate when a bitch is close to or at ovulation.
There is also an instrument, that reads the electric conductivity of the vaginal secretions and that is supposed to pinpoint the right time.