How are ejection seats tested?
Martin-Baker tests their seats in an actual aircraft. They have, for some time now, used a Gloster Meteor to test their designs as discussed in this question. They also have a high speed sled at their facility for such tests.. 0-0 tests are easier to pull off since there is no moving component.

Individual components like explosive bolts or handles are likely tested like any other engineering component, in a test facility.