Following are the guidelines if one is in doubt whether the correct version was said for parts of davening applicable at certain times during the year, such as morid hatal, ve’ten tal umatar, etc. [the prayers for dew and rain]:
If less than thirty days have elapsed since the change was implemented, we can assume that the previous text was said in error. If thirty days have passed, we can assume that the person remembered to say the new text, having already become accustomed to saying it.
In any case, a doubt is only considered a doubt if it occurs while the person is still davening Shemone Esrei. If at the outset one intended to say the correct version, and sometime after finishing Shemone Esrei begins having doubts, we can assume that the correct version was said, as was intended.
Rav Yosef Yeshaya Braun, member of the Badatz of Crown Heights. One Minute Halacha #156, reprinted from crownheightsconnect.com
