You will know when they're not playing.
Mating is very quick. The male mounts the female only for a few seconds then stops. He licks his shaft to clean and lube it up, then mounts the female again for another few seconds, then gets off. This process is repeated through the night the female is in heat. The females go into heat every 4-5 days.
It looks nothing like playing. A female in heat will hop around enticing the male as well. When she is receptive, she will wiggle her ears and arch her back to allow the male to mount her.
Overall, you probably wouldn't witness them breeding anyway. Peek breeding time is around twilight and throughout the night.
http://ratguide.com/breeding/figures...g_figure_1.php
http://img.photobucket.com/albums/v9...82535414_n.jpg