A negative number has it’s msb as binary 1 …Hence we rotate the number through carry to the left and then decide upon the status of the carry flag .
MOV A,#number ;The number to be checked for positive or negative
RLC A ;rotates the accumulator left through carry
MOV A,#00h ; moving 0 to show its positive
neg: MOV A,#01h ;moving 1 to accumulator to show the number is negative …you can do any other operation.