Sewer smell bathroom
We've moved into our 1st home August 1, 2013 about 1.5 months ago. The house was built in 1956 and it has city water and sewer.
About 2 weeks ago, we started noticing a "rotten egg" smell coming from the bathrooms. It is stronger in the downstairs bathroom but (according to my wife) can also be noticed in the 2 upstairs bathrooms. I am trying to figure out how proceed. Would like a sense of tips and tricks to diagnose this thing before we call somebody out.
From google I've learn that:
1) Could be a bad gasket in a toilet bowl connection to floor
2) A sewer backup problem.
One more piece of informaiton is that this smell was not there during inspection (of course...) or during 1st 2 weeks. But the weather was hot then and we had the windows open all the time. The smell has kind of coincided with the arrival of Fall weather (and consequently our closing the windows). Could be coincidence.
Nevertheless, could someone recommend 1st steps to diagnose this? We don't have water (from sewer) in the basement. We could buy sewer backup protection from Water regional company (annual) but I have no idea if that's a good idea/worthwhile.
Any advice much appreciated!