Real time PCR is the way to go.  Extract total RNA from your mouse cells using your method of choice (I've used the miRVana kit from Ambion/Life Tech before and it worked well).  Then do a poly-A reaction to add a poly-A tail to all the RNAs in your extract, then do a real time PCR using a poly-T primer and a primer that is specific to your miRNA of choice and you could compare to a miRNA with a known expression level.  Make sure to include a negative control as well to ensure that your miRNA-specific primer only amplifies your target.  This will help you quantitate the levels of your miRNA of interest.