Your mechanics may vary. For a 5-man crew, as with everyone else, that's going to primarily be the U's call. Our 7-man mechanics have the L holding the line, so that's his job to watch that.
On a related note, I spent most of this season downfield working B, F, or S, and had several flags for potential PI that I've thrown on screen plays. When I've been working B or F, before I even go to the R I've been heading o the L and asking him if the pass crossed the LOS. If not, I've been telling the R "I had contact beyond the neutral zone by A76, but the pass didn't cross the line of scrimmage. Wave it off." I'd rather have the flag down and wave it off than not call a clear violation.