Personally I like to use the 'Reps in Reserve' (RIR) model. For each set I ask my clients to record how many more reps they feel they could have completed in good form. Very simple and gives me an indication of how hard the client is working. This is vital when progressing the workout (increase load/reps/ROM ect.) or even more importantly can help to identity if the intensity is too high and increasing the risk of injury.
Maybe this isn't for every trainer but it would be nice to have the option to customise the workout information.
RPE scale is another simple option.