How do you Tell the difference between male and female deer mice?

We think we have a family of mice here, and we would like to keep them together. but we are not sure if they are male or female, how do we tell them apart so we dont end up with little ones...How do you Tell the difference between male and female deer mice?
Put them in a large box with a TV remote control in one corner and a credit card in another. The males will go to the remote and the females will head for the credit card.How do you Tell the difference between male and female deer mice?
If they have a bladder,or a peanuce
Rodents are easy to sex. Look under their tail if they have balls it's a male. If it doesn't it's a female. The ball sack is very large and very noticeable in mice. Don't put too males together they will fight. Females are OK together or a few females and one male. But if you mix females and males you'll have babies every couple weeks to deal with.