This is based on my P. cubensis hunting experience; hunting cow fields in Louisiana.
Best rule of thumb is that you need to wait till after the next hot day. There will be a few during the rains or even before, but not enough to spend your time for. Basically, what you are looking for is the first hot sunny day after a rain. that night will be the best pickings. In some cases, the fields will be too soggy. These scenarios require patience. Assuming you get multiple sunny days after the rain, go every night to check if the field is dry enough to fruit some mushrooms. During these trips to the field, you may find a few here and there. Remember where these singles are as theres a good chance the area will fruit abundantly after the field dries a little.