Identify a technique used to express or validate how hard a client feels when he or she is working during exercise?

Rated Perceived Exertion (RPE) Scale

Explanation:

Based on the information provided within the question it can be said that a technique or tool used to validate this is the Rated Perceived Exertion (RPE) Scale. This is a scales that is used in measuring a client's intensity of the exercises that they are doing. Usually measuring from various different intensity levels from 0-10 as seen below in the chart.

answer; generalizability;