Evert from Brownsburg, Quebec has the problem of a sewer odour in his bathroom but isn't sure where it is coming from and whether or not he needs to change his toilet.
It can in fact be difficult to localize the source of sewage odours and hence which fixture or drain is guilty. One technique is to thoroughly air out the bathroom to clear out the odours and then tape a garbage bag over the sink or the drain of the tub or a floor drain, or all of them. Leave them for several hours. Then one at a time, break open the sacks and stick your head in for a good smell. If there are no bad odours inside the bag, that fixture or drain is not guilty. This can often get you working on the right problem quickly. If none of the fixtures are guilty but the odour is in the bathroom itself, now you need to think about a possible problem with the plumbing vent system inside the walls, but at least you have carefully ruled out all the drains before opening any walls.