One possibility is to search for questions tagged by a given tag sorted by relevance. This will usually1 put questions with only this tag at the top. Example:
searching for "[calculus] is:question" and choosing relevance tab
https://math.stackexchange.com/search?tab=relevance&q=%5Bcalculus%5D%20is%3aquestion
There are also various SEDE queries for this purpose (for example, this one). However, data in the Data Explorer are only updated once a week.
I will also mention that there are some feature requests on meta.SE which ask for ability to search for questions having only a given tag:
1I did not find this documented anywhere on meta.SE, the only related question I found there was this one. When I try something similar on SO, I do also get questions with mixed tags. For example, when I tried searching for the tag php or javascript. I did not find a tag on this site for which I would get mixed-tag question on the first page of this search before single-tagged questions. (But I have only tried a few tags.)