I think we'll know with about a 90% degree of certainty what Justin Fields true potential is by the end of the season. If he doesn't start flashing more consistently with his arm or his brain by the end of this season, I think you can write off him ever being a "special" quarterback. Those guys just don't take that long to showcase their talent anymore.
Even with a guy like Josh Allen, who took 3 years to fully establish himself, his arm talent was on full display from day 1. He just wasn't accurate enough. I have only seen but a few glimpses from Justin Fields' arm that have wow'd me. His arm strength is very mediocre. I've never seen him make a pass and said, "wow, look at the zip on that ball!" It's just meh.
His inability to process information on the fly at the NFL level has held his pocket passing back greatly. Through 2 games, I've seen no improvement in that area from his rookie season. That is a major concern. This is the area where he's going to have to make up the most ground if he ever wants to be a great quarterback, but it's also the hardest to do.