First trick is determining the position in the NFL- if possibly a defensive end. Most 4-3 DE’s are fairly long and built, but some 4-3 DE’s in certain systems are “designated pass rushers” or “pass rush specialists”, that might come in on 3rd down or any obvious passing down. That is generally a smaller, speedier defensive end that could be a liability against the run.
Primary skills to evaluate:
Ability to drop into coverage (including covering matchup nightmares like speedy tight ends and running backs that move into the slot to receive).
Run stopping ability.
Blitzing ability (or general pass rush ability).
“Sideline to sideline” agility and range.
Can “break down”, which is a tackling technique of stopping before you tackle someone so you don’t get beat by last second moves.
Number of sacks / INT’s, TFL’s
Most of the OLB’s in the scouting reports are either middle linebackers in 4-3’s or OLB’s in 3-4… the skillset is often pretty close. Guys that are destined to play strongside or weakside in a 4-3 are generally a tier below and skew strongly to either better against the run, or better in coverage.