The Z/p^s-additive codes of length n are subgroups of (Z/p^s)^n, with p prime and s>=1. They can be seen as a generalization of linear codes over Z2, Z4, or more general over Z/2^s. In this paper, we show two methods for computing a parity-check matrix of a Z/p^s-additive code from a generator matrix of the code in standard form. We also compare the performance of our results implemented in Magma with the current available function in Magma for codes over finite rings in general.