
For a natural mating with fertile dogs a pre mate should be ok IF your vet is familiar with doing them and they have storage spot on.
I have run serum progesterones in addition to pre mates with my vet to test accuracy and they were really quite good BUT bad storage or poor operator technique can affect result. Given the cost is similar to lab serum progesterone I would use a lab test if possible BUT they can be handy for checks over weekends / BH