Two ways: firstly, known amounts paid to players pre NIL. I'll give you two examples:
The reported amount paid to Cam Newton, by far the most talented player in the country and single handedly won an inept coach a National Championship was reportedly $180k. That is far, far, far less than the amount of money going to top players today.
Other reports of impropriate pay comes from Laremy Tunsil at Ole Miss in 2016. He reportedly had coaches pay his families $300 dollar utility bill. He got suspended in 2015 for receiving a loaner vehicle for 6 months, got a $3000 down payment to help pay for a new car, and an airline ticket. He was a top NFL talent draft pick.
There are quite a few other stories of players receiving money like Tunsil, and the numbers aren't eye popping. The amounts, while not "small" aren't life changing either. While it's impossible to know the exact amounts players received pre-NIL,
LASTLY, my purely anecdotal experience as a student in college back in the late 2000s. A recruit, a player, and a bunch of people were at a party. The recruit asked for money and for somebody to go to an ATM and his player chaperone seemed to really be taken aback by the request. Like it was absolutely outlandish that this kid would be asking for money. Part of that could be because Bo Pelini policed his team hard and really pushed for no violations like that to occur. I also ate lunch and chatted with walk ons when it was mini-pizza monday at the Cather-Pound dinning hall, and they regularly complained about how most of the players couldn't afford to go out to eat and they were sick of the training table and that's why they ate at CPN. Mark Pelini, who has sadly passed away, also complained about how much the lineman had to eat was bordering on disgusting over the summer and he had a hard time keeping the weight the coaches wanted him to have.