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.
…a baraita was taught in the school of Rabbi Yishmael: A person should always converse euphemistically… And it says in another verse: "And you choose the language of the crafty" (Job 15:5), meaning that one should be clever when speaking and avoid inappropriate phrases. And it says in another verse: "My words shall utter the uprightness of my heart; and that which my lips know they shall speak sincerely" (Job 33:3). (Steinsaltz Talmud v6, Pesachim I, PI 3A / pg 13)
Some sages distinguish between [euphemism and dignified language]. One should go out of his way to speak euphemistically, so as to avoid using a crude phrase. Dignified language, however, is used not to avoid despicable language, but rather to avoid language that includes a slight trace of the derogatory or which bears a negative connotation, e.g. the direct mention of the night. One need not avoid that type of speech at the expense of brevity. (Steinsaltz v6 Pesachim I, p 13)
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:
The student who used the word impure spoke in a more concise manner, which the Gemara stated is appropriate for a teacher to his students. Nevertheless, a student may speak at slightly greater length in a more refined manner, in deference to his teacher. Furthermore, the use of the term impure is actually imprecise in this context, as there is no obligation to collect olives while ritually impure; rather one is simply not obligated to be in a state of ritual purity when collecting olives. (Steinsaltz vol6 Pesachim I, p 14)
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.