Well, what I used to do is look at situations where you have at least one older starting WR, and then a raw prospect sitting behind them, which the team is high on. Typically these guys come out of smaller schools, and have good measurables, drafted in the first few rounds of the draft. It helps to read the team message boards for a pulse on those guys, to see if they'll be able to make it or not. A lot of times people forget about these types of guys, especially if they are injured early on in their careers. Back when my dynasty league started up, the guys I came across were:
Some that panned out:
Brandon Marshall, drafted in the 4th round from UCF, sitting behind Rod Smith at the end of his career, and Javon Walker (didn't think he would fall off a cliff though). Once Rod Smith retired, and Cutler started looking pretty promising, I thought I had a 10-year guy in place; of course we all know what happened.
Vincent Jackson, drafted in the 2nd round in 2005 from Northern Colorado. He was stuck behind an aging McCardell and Eric Parker/Reche Caldwell. He was continually injured his first few seasons, but you could tell that the potential was there. (Once he started to take off, I thought I had another gold mine, but now with the holdout, it's looking like that's not happening)
Ones that didn't pan out:
Patrick Crayton, drafted in 7th round in 2004, behind Keyshawn/TO and Terry Glenn. I thought he'd be the one to step up after Glenn left, and I remember the buzz being pretty good about him, but he never turned the corner.
Robert Meachem, first rounder in 2007, and I think was injured the whole first season. Didn't do much his second season either, but last year had some success.
This may be WR 101 for a lot of people, but I thought it was a helpful way to think about whose number might be next.