Well, I have always paid attention to the way a male acts, because (ultimately) it is more reliable than a human-made blood serum progesterone kit, and it's free to boot

When Silverback was with a bitch not in heat, they were just buddies ...
When he would start licking them constantly, but wouldn't really mount, I knew they were coming in (or in) heat ...
And when Silverback could not stop mounting them, and would whine and get "pussy drunk" if I separated him, then I knew they were ready, no blood test needed, and he was always right

Jack