Have a strong foul smell in bathroom. Put a bag over the sink and tub drains and am still getting smell. This leaves the commode? How can I check my backvent(is this correct term)

A clogged usually announces itself by a gurgle from one of the fixtures. There are a few things that would cause odor in a toilet.
1) a ruptured wax seal
2) A lowered water level in the toilet bowl
This happened in my master bath last week. I had a bad smell in there and thought I had a ruptured seal until I noticed the bowl level. The little tube that runs from the ballcock to the white overflow tube had moved out of position and was no longer helping to fill the bowl. I replaced it and the stinky went bye-bye. If your levels are OK then replace the wax seal. Good luck. Tom