Ambiguous question. Will the cell in A contain only a single letter or a string of one or more characters? Should the formula detect only lower case x, upper case X, or be case insensitive?
Here's a general solution:
Formula in all cells in column B: =IF(OR(LEN(SUBSTITUTE(A,"x",""))<LEN(A),LEN(SUBSTITUTE(A,"X",""))<LEN(A)),"X"," ")