First, you need to look up the Instruction Set definition.
There's a good one in the Philips 80C51 Family Programmer's Guide and Instruction Set manual:
http://www.semiconductors.philips.com/acrobat/v...UIDE_1.pdf

This will tell you how the machine code hex instructions are formed from your assembler source.

Then, you need to read the Intel Hex file format spec (already cited in this thread) which will tell you how the object code is represented in a hex file.