IMO the general condition is you have to have been sustained top 2 at your position for your career/era. The secondary condition is you were top 5 at your position for a long time and racked up stats. WR is tricky because they're still calibrating what counts, in terms of both peak and career. And it's so biased by what QB and OC a guy played with. Edge and Gore's RB numbers will likely stand the test of time, so that will help. It's incredible to think that the '01 team had > 30K future NFL rushing yards on it. And over a three year span to include Edge (and others), it's more like 50K.
Wilfork is the most interesting guy, because he really may be the best ever NT. That plus SBs will get him respect. May take time but wouldn't bet against him.
Guys like Olsen, Campbell, these guys are not Canton level, IMO.