As an Indian I am familiar with the social and cultural circumstances that make it difficult for you to ask your future wife the direct question on such an intimate subject. However I would suggest that a lot would depend on whether your marriage is an arranged marriage or a love marriage.
If it is an arranged marriage then you are very right to be apprehensive that your question might be badly received and misunderstood. If the girl comes from an orthodox family and happens to have 'strict' opinions about such matters it might even endanger the marriage-engagement. So I would actually advise you to even ask the question only if you are very sure that she will not misunderstand you, and I strongly suggest that you be very cautious here. You can still ask the direct question if the topic of sex comes up naturally during your discussions, as noted in the earlier answer by @AJ. You would feel more confident if you and your fiancee have been interacting a lot after your engagement, have formed a good rapport with each other and are comfortable with talking about such sensitive topics. Or else you can try to assess her overall attitudes and opinions about such matters and decide when you can 'safely' ask the question.
However, if it is a love marriage (as appears likely since you refer to her as your girlfriend) then you have much more license to ask her outright if she will like to perform the particular act. You need not hesitate to ask, because you are already in a relationship, and physically intimate with her to a limited extent. May I only caution you that you need to anticipate she may not like to do it and it is her right to dislike any particular act. So you can think in advance how her possible refusal would affect your marriage outcome.
In either case, assuming you are able to be comfortable with asking her, it's always better to ask the direct and explicit question because indirect phrasing or euphemism carries the risk that if the person does not understand what you are asking, she might possibly say "yes" as many of us Indians tend to do, and it could lead to a great deal of confusion in your future married life. You should also be careful to avoid insulting her: be sure you don't unintentionally imply that she has any previous experience of such matters, as pointed out by by user @AllTheKingsHorses in a comment.
Further, based on the clearly expressed understanding that you and your fiancee shall try anything only after marriage, you can take @Kate Gregory's wise advice as stated in a comment here:
rather than asking her if she would like to do it, ask her if she would like to try doing it with you. This is more specific, doesn't presume she has done it before and knows what it's like, and emphasizes that it would be as part of your relationship. She may be willing to try it and discover she doesn't like it. That's not the same as not being willing to even try it, right? You two can learn how to do things like this together.
However, her reply (if you ask her before marriage) could vary from "willing to try after marriage" to "will not even consider the idea" depending on which you can decide whether or not it would be a deal-breaker, and the two of you can take a final decision accordingly.