Suppose that one is not sure whether a certain phrase is considered Nivul Peh or not. Could it be Nivul Peh to ask a Rov if that phrase is Nivul Peh (using the phrase)?
I was just reading Pesachim the other day and it discussed the issues of euphemism and dignified language. I would interpret the examples to mean that yes, it is nivul peh to directly use the phrase to ask if it is prohibited, although you do want to balance these concerns with the need for brevity.
Pesachim I opens with a mishna in which Yehuda HaNasi uses "or" (light) as a euphemism for darkness / night, and true to Talmud form, there are about 8 pages of exegesis on whether and why "or" is to be taken literally or as a euphemism.
There are more examples that try to establish the balance between brevity and dignity of language, which seem to favor dignity e.g. this interpretation from Steinsaltz about one of the examples:
So you should be both euphemistic and brief in asking such a question, but favoring dignity and not using a crude term. Like, suppose you were asking about whether the word "pig" were nivul peh. You could start by phrasing "Rav, I wanted to know if a certain term is nivul peh," to indicate that your inquiry is in the spirit of knowledge and you're not trying to be crude. Then you say "the term is the p-word," and use as euphemistic a term as possible to avoid speaking crudely. If the Rav doesn't know what you mean you provide a little more detail until the term is clarified.