Well to be perfectly honest I don't know. I bought all the mice from a woman who seemed very experienced in breeding mice and rats and told her what I wanted and she put them in 3 different boxes in the colonies they were intended to be permanently, so I guess I just trust her.

I've seen the same vid from RDR, however that's rats and it feels like it's easier "popping" rats than mice thanks to the size difference. I'll give it a gentle go at next cleaning time and then check genitalia distance etc. My biggest problem is, I can't pick one up, check him and then put him/her back in and take another one. Simply because they are all labb mice and are all white with red eyes, there is absolutley no way to tell a difference between them so I might just pick the first one up 3 times or something.

Cleaning day is tomorrow, I'll update after that. Thanks for all the replies.