How will I know if my child has passed the retained stool?

[marooned]

My almost 5 yr. old has been taking Miralax since he was about 2 yrs. old. for stool holding.  I took him in for his 4 year check-up to see about getting him off the Miralax since I thought he was doing good with his toilet time.  So the Dr. ordered an abdominal x-ray and found that there is a large retained stool mostly in his rectum, I believe.  This was hard to believe since he goes poop daily.  But he said that it is leaking around the hard stool.  Anyway, we tried glycerine suppositories for 3 days and that didn't seem to do much.  He started complaining of tummy aches from the start of that so I took him in and the doctor told me to try Dulcolax suppositories for 3 days and if that didn't work to try an enema.  My question is this:   How will I know if my son passes the retained stool?  How big are they usually?  We just finished the 3 days on Dulcolax and I haven't noticed any real huge poops except for on the last day after giving it to him.  He went shortly after and the poop was more formed than usual and had a different look than normal...kind of looked like something old.  I guess I'd consider this a large poop but how do I tell if this is what needed to pass or if he needs an enema.  He still complains of tummy aches but I have read on the Dulcolax box that this is common while taking the medicine.  (we just finished the dosage today)  Any information or insight would be helpful and appreciated!  
Thanks!
Amy