Q. If one has a doubt if he davened on Shabbos, or if his davening was done correctly or for whatever reason may have been done incorrectly, can he eat? There is no solution to say a voluntary prayer on Shabbos.
A. Horav Shlomo Miller Shlit’a suggested he should ask the shaliach tzibur to have him in mind during the amida repetition and comply by listening to it. The Rov stressed, that the prohibition to eat before davening applies only if one has to daven. When one is exempt, as in our case when out of doubt one is exempt of praying, one is permitted to eat the seuda of Shabbos.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a