Is it permissible to fast days with the intention of [resolving] doubt about the number of days that must be made up?

Whoever missed days in Ramadan and does not know their number must fast until they are reasonably certain that their obligation has been fulfilled. If a woman fasts until she is reasonably certain that she has made up what was due from her, then nothing further is required of her. Some scholars hold that one who doubts the necessity of making up (a fast) is not required to make it up, because the default is the exoneration of one's responsibility. The more cautious approach is to make up this doubtful day. Thus, her responsibility will be definitively cleared if it was obligatory, otherwise, it will be a voluntary act.