Phosphatases can be assayed by measuring phosphate (Malachite Green for instance). Hexokinase can be coupled to G6P dehydrogenase and monitored by absorbance at 340nm. For the GPI assay, the approach will depend on the actual reaction you want to measure... It is easy when converting F6P to G6P (coupled with G6PDH like for hexokinase assay). A bit more tricky the other way around...