I know this one is not a new one but as I have not received any answer
Shall I say
If I was a detective, I would ask or I would be asking
Is the second one (be asking) only a form to stress politeness
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Both are grammatical, but "I would ask" sounds much more natural. "I would be asking" simply sounds awkward (since asking is generally a very quick action and is not done progressively). The progressive might be acceptable if you plan to ask questions for a long time:
I would be asking all of the neighbors a lot of questions
This roughly means:
I would spend my time asking all the neighbors a lot of questions
If you want to stress politeness, you could add uncertainty by saying "I might ask".
Note that this is a counter-factual conditional (i.e., you are not actually a detective), so the subjunctive were is appropriate:
If I were a detective...
As written, "If I was a detective..." means "If, at some point in the past, I was a detective..." which you might ask if you had lost your memory and could not remember if you ever worked as a detective:
I don't remember clearly, but I think I might have been a detective before I got amnesia. If I was a detective, then someone at the police office might recognize me.
I do not see the difference as pertaining to politeness or a manner of speaking. To me, it is more like the following: