checking for sperm plugs?

I'm just curious. I'm almost afraid to ask this. How does one check for sperm plugs? The males I've bred in the past were at least a couple years old. Their sexual maturity was never a question. I see "they are producing sperm plugs" alot in adds. Just wondering how they know.

You basically pop them out of the vent area, like how you pop for sex.

They come out pretty easily. I think this was discussed in a recent thread as well.