2000 fuel gauge or sending unit problem ?

[rigid]

2000 suburban . Had a new fuel pump module , which includes sending unit replaced at GM dealership lifetime warranty if done at the dealer . less than a year later gauge starts dropping and reading empty . Does this a few times and has been like that since . How do I determine if it is the gauge or the sending unit ?
Stealership and Gm say the new sending unit was not the pump so it is not covered . Their contention was that the pump still works .
Thank you