How to know the gestation time of pregnant mice?
This is generally done by checking for the vaginal plug in the female after you have set up the breeding. If the plug is visible then it means that copulation has happened. However, this is not a foolproof test because the vaginal plug just indicates copulation and not pregnancy. This, nonetheless works in most cases and helps you in timing the pregnancy (Reference: JAX notes).


  For most strains, the rate of pregnancy is highest among
  estrus-suppressed females with vaginal plugs found the third day after
  being placed with a male. As examples, whereas the rate of pregnancy
  is ~100% and 44% respectively for C3H/HeJ and BALB/cJ females with
  plugs found on the third day, it is only 62% and 31% respectively for
  those with plugs found on the fifth day. In contrast, whereas the rate
  of pregnancy for C57BL/6J females is only 39% for those with plugs
  found on the third day, it is 69% for those with plugs found on the
  fourth day.
  
  If the plugged female is pregnant, the first day of gestation is
  considered to be the day after the plug is found.


You can also monitor for an increase in weight of the mouse. This is not a foolproof technique but serves as a secondary evaluation method. 

If you have not timed your breeding and checked for these markers, then you may have to rely on ultrasound.