Answer:
Some signs are;
-Mounting of the same sex, the mouse mounting the other is showing dominance.
-Chasing, the dominate one will chase the other one, often the dominate one may also 'hold on' to the mouse in front with its mouth/teeth on its back near to the tail.
-Over grooming of each other (Called barberism), can also be a small sign of dominance, you may notice the dominate mouse over grooming the other, leaving patches in the others fur. But don't worry, it's not a painful condition for the mouse.
-Standing up on her hind legs to the other mouse, is a sign she is willing to fight, often the other mouse gives in and a fight will not usually break out.