How can a stylist determine if a hair relaxer is effective before applying it to the entire head?

A stylist can determine if a hair relaxer is effective before applying it to the entire head by performing a strand test. This test involves applying the relaxer to a small section of the hair to evaluate its texture and response to the chemical treatment. The strand test is crucial as it helps to identify the appropriate processing time for the individual’s hair type, assess the hair's health, and predict the overall outcome of the relaxer application. By observing how the hair reacts, the stylist can make necessary adjustments to ensure optimal results when treating the entire head, reducing the risk of damage or undesired effects.

The other choices do not provide a direct assessment of the relaxer's effectiveness on the client’s specific hair. Checking the expiration date ensures product safety but does not indicate how well the relaxer will perform. Consulting the manufacturer’s manual can provide guidelines but may not account for individual hair characteristics. Looking at the client's hair length does not provide information about the hair's condition or its reaction to the relaxer, making the strand test the most reliable method for this evaluation.