If your 172 has the words Not Certified for IFR flight, Certified for VFR flight Only, IFR flight not authorised, or VFR Only (in the operating limitations) or other similar language then it is not certified for IFR. If there is no restriction in the POH/AFM then there is no restriction.
To expand on Bookworm's post, the equipment requirements for IFR (Class G) are minimal (altimeter, gyros, a time piece, thermometer, and rate of climb indicator) beyond those required for day VFR. For Class D a radio, transponder, dme, vor are required as well),