I have one bathroom where the hot shower water has a sulfur-like odor to it on most days and I'd like to have someone come and check it out but I'm not positive on who to call.
If you are on a public water supply, there may be water test results already published on a quarterly basis.
If you are doing your own testing, generally an environmental testing company or consultant (rather than a plumber) - in many cases you can save money if you can follow directions carefully and take the samples yourself, for testing at their laboratory, rather than having a consultant come and take the samples for you.
If it's ONLY the one bathroom, it's more likely something in your pipes than in the general water supply.
As @tyler durden comments, this may be an issue with the drain, but it is very unlikely that there is no trap. A more likely scenario is that a partial blockage or obstruction of either the drain line or a vent line is causing a siphon action that is preventing the trap from maintaining a water seal. Another possibility is that one or more drains have been added (or the drain configuration has been changed), causing inadequate venting and/or siphoning.
Sulfur smell is often caused by degradation of the sacrificial anodes in the hot water heating system.