If one forgot to say Mashiv Haruach:
Nusach Arizal/Sefarad: Those which Daven Nusach Arizal or Sefarad do not go back in the prayer if they accidently omitted Mashiv Haruach, as in any event they are accustomed to say Morid Hatal in the summer days that Mashiv Haruach is not said, and this mentioning suffices in place of Mashiv Harucah. Thus, even if one did not yet finish the blessing of Mechayeh Meisim he is not required to go back.
Nusach Ashkenaz: Those which Daven Nusach Ashkenaz, if they omitted Mashiv Haruach, then if they already concluded the blessing and began saying the next blessing [even the first word of “Ata”] they must return to the beginning of the prayer, irrelevant to where in the prayer they remembered of their omission. Similarly if they remembered only after concluding the entire prayer, they must repeat the Davening. If one remembered prior to saying Hashem’s name in the blessing of Mechayeh Meisim, then he is to say it in the place that he remembers and continue from where he left off. [If he already said Hashem’s name then he is to finish the blessing and say it prior to beginning the words of the blessing of Ata Kadosh. The same applies] if he only remembered after concluding the blessing of Mechayeh Meisim but before beginning the next blessing [even the first word of Ata], then he is to say it there. If however he already began even the first word of the next blessing he must return to the beginning of the prayer, as explained above.
If one is in doubt if he said Mashiv Haruach:
Those that Daven Nusach Arizal or Sefarad and recite Morid Hatal during the summer, are never required to go back even if they certainly did not say Mashiv Haruach, and certainly if it is a mere doubt. However those which Daven Nusach Ashkenaz and are in doubt if they mentioned Mashiv Haruach are to consider it as if they omitted it for the first 30 days after Shemini Atzeres [until 22/23 Kisleiv], and hence follow the same ruling as one who omitted it.
 Meaning even he did not yet say Hashem’s name in the concluding blessing.
 Admur ibid in parentheses; Implication of Beis Yosef 114 as brought in P”M 114 A”A 7; See Chikrei Halachos 7/11 that this matter is in truth unclear from the Poskim, and hence Admur placed it in parentheses.
Other opinions: Some Poskim write that according to the Michaber one is required to go back so long as he did not finish the blessing. [P”M ibid]
 This depends on if we count Shemini Atzeres as the 1st day or not. Vezatrcuh Iyun.
 Prior to the conclusion of the 30 days one is not to repeat the beginning of the blessing of Michayeh Meism until … Morid Hageshem for a total of 90x in order to make for oneself a status quo that he has now regulated himself to say it and hence no longer needs to go back in case of doubt, as there are opinions which argue on this ruling and hence one ends up entering himself into a dispute if he does not remember if he said it prior to the 30 days concluding.