Hi rock,
Yes it is mandatory for a general category student to show the family income certificate with a stamp of tehsildar on it as it is being verified for sure .
This verfication is done during your josaa admission round when you had finally selected your college and then now you are about to verify all your certificates to them .
There is no other way a general student can get fee waiver in nits .as there are no scholarship test during class hours for the deserved candidates .

Hope it helps !!!