Text matching as @SqrtOfPi is probably the easiest way, but you could conceivably use simpleFunction which would have the benefit of working even if there were extra spaces. Something like this would be correct with either capital or lowercase ‘a’:
correctA = isDefined(simpleFunction(this.cellContent(1,1),"A","a").evaluateAt(1,1))
Obviously you’d need to change which cell and the letters.