The ability to identify zero variables early on in an iterative method is of considerable value and can be used to computational advantage. In this work we first give a formal presentation of the notion of indicators for identifying zero variables, and then study various indicators proposed in the literature for use with interior-point methods for linear programming. We present both theory and experimentation that speaks strongly against the use of the variables as indicators; perhaps the most frequently used indicator in the literature. Our study implies that an indicator proposed by Tapia in 1980 is particularly effective in the context of primal-dual interior-point methods.