The plasma-surface contact in low-pressure rf discharges in noble and reactive gases was studied by a combination of probe measurements and ion energy analysis at the surface. By comparing different methods, as the most reliable density determination the integral of the second derivative of the probe curve was found. This method turned out to be not very sensitive to incomplete rf filtering or compensation and even to coating of the probe. However, it depends strongly on the correct choice of the plasma potential. By comparing space potentials obtained from probes and from ion energy distributions, a most probable value can be attained that is situated between the negative minimum and the closest maximum of the I"(U) curve. The ion energy distributions are compared to an analytical sheath model, giving very good agreement in an ab initio comparison. In plasmas depositing carbonized polymer films, reproducible structures due to the influence of the coating on the probe characteristics can be analyzed, yielding nonlinear I(U) characteristics of the film. Using these characteristics, the film growth can be followed.