How to know teeth under bridge have decayed - I never had pain/sensitivity until porcelain chip off from middle unit. (Photos)

After have reviewed your two X-rays, the 15 year old bridge need to be replaced. The two teeth your old bridge sits on are no longer the shape that the bridge was once made to fit them. Decay may have eaten your tooth structure away under the bridge where now there are “gaps” under the bridge. We call that “open margins” in dentistry. Think of it as not sealed 360 degrees around each tooth. The pain now on tooth #13 may contribute to decaying process into or near the nerve (pulp). A root canal therapy maybe indicated to resolve the pain prior to your new bridge placement. Your dentist will test to see if a root canal therapy will be needed. Another recommendation, there’s always a reason why an old crown or bridge dislodges and recementing them back on is never a good idea. When you do that, you chance cementing bacteria or decayed structure underneath the old dislodged bridge or crown. Trapping bacteria inside the tooth. Your bridge is over 15 years old, It was time for a new one back when it fell off in 2013.

It appears your either have open margins or decay under the crowns. There's no way to tell the extent of the decay if it is decayed without removing the crowns from those teeth. That would mean redoing the whole thing. 

Hi:The crown next to the cateliver (tooth #13) seems to have an  unsealed margin or decay.  It is difficult to tell without removing the bridge.Best,Dr. Maddahi