1
This will give you True/False, though you can nest it in an IF statement to get Yes/No.
=NOT(ISERROR(VLOOKUP("b",INDIRECT(ADDRESS(MATCH(1,A:A,0),2)&":"&ADDRESS(MATCH(2,A:A,0)-1,2)),1,0)))
The variables are "b" which is the letter to look for, the 1 in the first match statement which is...