A selection is not a reach if the player would not have been available at the time of the team's next pick. There is no way to determine this, therefore, there is no way to determine whether or not a player was a reach. To do so is stupid, immature, and will resort to name-calling and a general feeling of animosity and disgust.
You can, however, debate the sensibility of that particular selection, and whether or not it was a good pick based on the actual player. There is a reason teams go into a draft saying "a draft is a success if you get [insert number of preference] contributors," regardless of where they were drafted.
The inverse of this (the "value" pick) is just as stupid, especially so when immediately following it they attempt to label the "reaches." If a player falls it is because that is where the player was appraised to be worthy of going. It can be a good pick, or a value pick for a team based on talent,fit in system, and necessity, but it is not a "value" pick in terms of draft position.
SI labeling their "value" selections is just them skirting the admittance that their original rankings were wrong, and that all of the NFL appraised that player to be less than how the "consensus" draft-geek rankings previously rated. Because they already screwed up big time, don't point to SI as any sort of justification of your belief that a pick was a "reach," because it's just as likely that their designation of a reach is a screw up as well.