I've done plenty of procedural work in my time. If you issue clearances that are sound - i.e, clear and provide an appropriate separation, but the aircraft do not comply with the clearances then there is a possibility that an unsafe situation will develop. In a procedural environment, and without the pilot advising that he/she cannot comply with the clearance, the only way this is likely to be identified is if the controller picks it up from pilot reports - or if one pilot reports that there's another right in front of him or whatever! That's life doing procedural control.
If you are worried about this I guess you can issue sound clearances and ask for progress reports every minute from every aircraft that you're working - but as already pointed out, this does rather defeat the object of issuing sound clearances in the first place. Doing procedural control one will - or should - always be alert to cues that might suggest that separation is not being achieved, but I must re-iterate, if the clearances are sound then the only way it can fail is if the aircraft do not comply for some reason. And surely in normal operations we have to trust what the pilots tell us.......