  In both Newtonian gravity and Einstein gravity there is no force on a test
particle located inside a spherical cavity cut out of a static, spherically
symmetric mass distribution. Inside the cavity exterior matter is decoupled and
there is no external field effect that could act on the test particle. However,
for potentials other than the Newtonian potential or for geometries other than
Ricci flat ones this is no longer the case, and there then is an external field
effect. We explore this possibility in various alternate gravity scenarios, and
suggest that such (Machian) external field effects can serve as a diagnostic
for gravitational theory.
