There are two ways to go, ranging from a cheap, simple hand-held type just to take a fix reading (usually for proof) to the constant monitor types (i.e. by Minut) to monitor from afar (via internet) and made to order for STRs, it is a monthly fee type though.
A rough reference of noise (measured by DB) is: in the 50's is acceptable (normal conversation), in the 60's it is loud, in the 70s it is annoying and beyond it become unacceptable. Roughly every 7db is doubling of noise. Concerts usually are in the 100+ range and NFL games and some stadiums beyond that, just to give you an idea.