Signature is used to identify the chips make and its part number it is read in the same way like normal code varification the only difference is p3.6 and p3.7 must be pulled low for signature verification. for atmel parts such as AT89C51 and AT89C52 the signture is read from location starting 30H.

for example for
30H = 1EH Indicates that chip is atmel make.
31H = 51H Indiactes it is AT89C51
31H = 52H Indicates it is AT89C52
32H = FFH Indiactes 12V programming
32H = 05H Indiactes 5V programming.