You can purchase O2 indicators (turn pink in the presence of O2 and are colorless in the absence of O2) to help you determine when atmospheric air has been purged from whatever you are gassing.

As to liquid N2, I really do not know how well that would work, as is would freeze any item with which it comes into direct contact. If you had it in an open container within the container, which you wish to purge and seal it might work, but you could not seal the container until it, all of the N2 enter the gaseous phase, otherwise you run the risk of an explosion. Be very careful with N2, it can cause some very nasty burns, full thermal (long gloves, apron and foot wear designed for use with liquid N2) and eye protection are required to prevent splash injuries. Likely better off with a cylinder of N2 gas for this application.

Pete