It is brought down in mesheces Pesachim yerushalmi(perek 10,halacha 1) that Rebbi would fast erev pesach. There is a machlokes in the Yerushalmi whats the reason Rebbi fasted on erev pesach.One held Rebbe fasted because he was a bechor, and the other held because he was a istainus and wanted to be able to eat the matzah b’teiavon.
Q:Why couldn’t they just see what he did when erev pesach falls out on shabbas and the fast of the bechor is on Thursday?
A:It could be Rebbi held that when erev pesach is on shabbas there is no fast of the becohrim at all (see bais Yosef OC siman 470)It could also be that Rebbi even fasted on Friday like every other Friday(like the Shulchan Aruch OC 248:3) to go into shabbas able to eat. So one could not tell.
A:According to The Chida(Berchei Yosef 470:4) he held when erev pesach is on shabbas the bechorim fast on Friday,and therefore it would seem maybe Rebbi fasted every Friday because he was a istainus.Therefore it remained a safek why he fasted.