Is it possible to assess integrity, depth and loading capability of existing concrete piles?
If the existing concrete pile is drilled, then a low strain integrity test (PIT) might be an option to determine the pile length and integrity, as long as the pile is not supporting a structure. If the pile is integral in the structure then the chances of success are reduced. If the pile has substantial non-uniformity (neck or bulge) then seeing below that first major non-uniformity is generally not possible.
A parallel seismic test is an option to determine unknown length, but it requires drilling a cased borehole near the tested pile and cannot evaluate integrity.
If the length can be determined, assuming you know the soils you might be able to estimate the capacity from a static analysis.
If the pile is a one-piece driven pile, PIT or parallel seismic might be possible. If it is a jointed segmental driven pile, then PIT is not likely to be successful as the first joint usually blocks the wave transmission.
With either drilled or driven concrete piles, a high strain dynamic test (PDA plus CAPWAP® signal matching) would be an option to determine length and capacity (and also integrity for a driven pile). However, for a successful high strain dynamic test the pile top must be hit axially with a relatively large drop weight (say minimum of 2% of the ultimate capacity that you seek to prove).