The head coach is still learning about his roster, having only joined the organisation at the start of the year.
Marrone told the team's official website: "The critical time will be when they put the pads on and see what they can do making the tackles and blocking and reacting.
"It's very difficult to judge when they're never in the mode of full finish blocking. You never know who is going to turn it down a little bit, who is going to turn their head and who is going to use the proper technique.
"The thing I talked to the players about [during the off-season] is with a new staff and a new team is we're putting people in different spots. No one is going to make the team now. One thing players can earn is reps from their performance in the OTAs. Those reps will be awarded in training camp."
The Bills will begin their training camp on July 22.