  We analyze the possibility to measure small thermal effects in the Casimir
force between metal test bodies in configurations of a sphere above a plate and
two parallel plates. For sphere-plate geometry used in many experiments we
investigate the applicability of the proximity force approximation (PFA) to
calculate thermal effects in the Casimir force and its gradient. It is shown
that for real metals the two formulations of the PFA used in the literature
lead to relative differences in the obtained results being less than a small
parameter equal to the ratio of separation distance to sphere radius. For ideal
metals the PFA results for the thermal correction are obtained and compared
with available exact results. It is emphasized that in the experimental region
in the zeroth order of the small parameter mentioned above the thermal Casimir
force and its gradient calculated using the PFA (and thermal corrections in
their own right) coincide with respective exact results. For real metals
available exact results are outside the application region of the PFA. However,
the exact results are shown to converge to the PFA results when the small
parameter goes down to the experimental values. We arrive at the conclusion
that large thermal effects predicted by the Drude model approach, if existing
at all, could be measured in both static and dynamic experiments in
sphere-plate and plate-plate configurations. As to the small thermal effects
predicted by the plasma model approach, the static experiment in the
configuration of two parallel plates is found to be the best for its
observation.
