I'd say
Does 'when' function as a conjunction there? , or Does 'when' act as a conjunction there?, or Can 'when' act as a conjunction? - depends on what you want to say.
Using
to be you can say
Is 'when' a conjunction (there)?, or
Is 'when' a conjunction or an interrogative (pronoun) [or anything else]?
Is "when" act like a conjunction? doesn't work at all - there are two main verbs in one and the same clause (sentence)- that doesn't work, it's wrong. Maybe you can say
Is 'when' acting as a conjunction (there)?
Forming wh-questions:
With an auxiliary verb:
In English we usually form wh-questions with wh- + an auxiliary verb (be, do or have) + subject + main verb or with wh- + a modal verb + subject + main verb
Without an auxiliary verb:
When what, who, which or whose is the
subject or part of the subject, we do not use the auxiliary. We use the word order subject + verb.
Forming yes-no questions:
With an auxiliary verb:
We form yes-no questions with an auxiliary verb (be, do or have) + subject + main verb or with a modal verb + subject + main verb.
Where there is no auxiliary verb be, have or modal verb already present in the statement, we use the auxiliary do, does, did: Statement form (no auxiliary)
Without an auxiliary verb:
When we ask yes-no questions using the
main verb be, we don’t use an auxiliary verb. The word order is: be + subject.
Forming alternative questions:
An alternative question gives a choice of two or more answers in the question and includes or.
With an auxiliary verb:
We usually form alternative questions with an auxiliary verb (be, do or have) + subject + main verb or with a modal verb + subject + main verb.
When we ask alternative questions using the
main verb be, we don’t use an auxiliary verb. The word order is: be + subject.
By the bye, cordial welcome here.
