The frame is a good place to start. In the 6'4"-6'6" or so range. We like arm length particularly. Crucially, does the frame look like it can support muscle mass?
How big is the kid? If he's a "skinny" 280 or so, then a collegiate strength coach can put 30lbs of muscle on him in a year. Some 290lbs high school players have lived in the gym, and are at their maximum muscle build.
"Get off" speed: How quickly does he get from "hand in the dirt" to "set"? LOS battles come down to who gets their hands placed first. The DL wants to get in tight, grab jersey, and throw the OL out of the way. The OL wants to keep their interior clean by getting hands high and arms extended (see arm length above). Fast twitch muscle fibers are a thing. I can't grow or develop those. A kid is either quick out of the blocks or he isn't.
Demeanor: OL is an ugly, painful, thankless position. Your name is only called when you screw up. However else the kid lives, when he gets between the lines of a football field, I need him to enjoy shoving people over. Angry people still win football games. After your initial assignment, work to the next one. Look for opponents to flatten, and keep doing it until the whistle dies out.
I haven't said much about mechanics, and OL is all about mechanics. I'm taking it as accepted that the kid will have a decent base (feet squared underneath the body) and have an idea about how to move. An OLineman must move through an unpredictable tangle of bodies while keeping his feet on the ground underneath him for as long as possible (even though this sounds mutually exclusive). This is where a great OL coach can turn a 3-star project into an All-American. Foot technique. Balance. Leverage. Hand placement. Committing these to muscle memory so they happen without thought sets the player up for success.
A safety that exceeds his body type can be a LB. LBs can spin down to DE/DL. Since OL players don't usually get lighter, there's no real place for them to go. Committing scholarships to 16-17 year old males who you hope grow into useful OL players is definitely a voodoo science.