Why are so many tail-strike incidents not verified until later inspection? One often reads of a "suspected tail-strike" at take-off (for example suspected because a controller has seen something from the tower, or because a cabin crew member sitting at the back of the plane has felt a jolt or heard a noise).

What puzzles me is why this must remain merely "suspected" until an inspection.

As an example, a 737 suffered a tail strike that appears to have at least been considered a possibility by the crew. Apparently doubting that it actually had occurred, the flight crew continued to their destination; had there been a clear indication, they presumably would not have. 

Aircraft are festooned with external sensors and measuring devices; I would expect aeroplanes susceptible to tail strikes to have sensors measuring movement of, deformation of or contact with a plane's skid plate, to provide a cockpit alert, so that there need not be any doubt.

Since this often doesn't appear to be the case, there must be good reasons why such systems are not usefully employed - what are they?
On one hand tail strikes are mitigated long before the need for a sensor. Aircraft makers generally address it from a design standpoint before the aircraft ever flys. If that is not enough control augmentation or tighter procedures are developed to prevent it from an operational level. 

Ultimately such sensors do exist and do warn the crew of a tail strike they may just not yet be certified or installed on every airframe. For example it came in handy in this incident: 


  After the passengers had disembarked, the captain conducted an
  inspection of the tail skid of VZR and noticed some paint damage and
  scrape marks, however the cartridge containing the sensor for a
  tailstrike was still intact. This indicated that the tailskid had only
  just contacted the runway during the take-off.