Goals: What are your goals?
Many people hiring a personal trainer are interested in body recomp or fat-loss. Some of those people want to work on a particular feature. There are many who want to be stronger and feel better about their health and better about themselves in general.
Understanding goals and even unintended benefits of fitness programs and routines can help you find ways to "measure".
Fitness effects: mindset, attitude, fitness level, weight, size, strength, endurance, wellness...
Feedback and progress
It informs the client and personal trainer
Ways of measuring that don't involve a scale or caliper:
Number of Reps/ min
Number of Reps (in no given time)
1 Rep Max
Stop time: relays, planks (an other holds), mile run
Flexibility/ Range of motion
Pictures- can show changes in appearance overtime, but also can show clients improved form in their movements
Miles/ days worked out- habit trackers!