WR is one of the hardest positions to predict. A player can absolutely dominate the collegiate ranks but then they get to the pros and it doesn't work out.
When I started really studying for the draft I learned quickly that WRs are risky. The 2001 draft being the perfect example. That season David Terrell and Koren Robinson were taken 7th and 8th overall. Both guys were seen as safe picks but they never lived up to the hype.
If I was scouting a WR I'd focus on his work ethic. Obviously the height/weight/speed stuff is important but most of the elite WRs work really hard. They want to be great. So I'd look at that, does the guy want to be great. It's not something you can measure though.