First time visiting a 4-plex...suggestions?

[Nicholas Parish]

Hello all! First time posting here.

As the title suggest im looking for some advice on seeing my first 4-plex. 

I am a commercial roofer by day so i know how to spot roof problems. Old windows are easy to see as well. But what im not totally sure of is plumbing. I visited a duplex very recently that had water stains on the ceiling on the bottom floor. Which to me suggested plumbing issues. That one seemed easy but im not entirely sure how to spot the not so obvious plumbing concerns. Also ive never lived in anything other than a SF home so im not really sure what kinda problems a 4-plex could have. Any advice is welcomed. 

I started to post this on reddit but Brandon Turner has really  been helpful for me so i decided to come here.