The pre-service inspection: what a good beautician checks before opening the kit
Watch what happens in the first ten minutes, because that is the diagnostic phase, and skipping it is the clearest early warning you will get. For colour, the professional standard is a patch test of the actual developer and dye behind the ear or on the inner arm 48 hours before application. PPD allergy reactions range from an itchy hairline to facial swelling that needs emergency care, and prior tolerance is not a guarantee, because sensitisation builds with exposure. Any vendor willing to apply permanent colour to a new client with no patch test and no questions asked is telling you how they handle every other safety step.
For nails, the equivalent inspection is of the nail plate and surrounding skin. Greenish discolouration under old gel, thickened or crumbling nails, or broken skin around the cuticle are all stop signs, some of them fungal or bacterial, and working over them seals the problem in. A conscientious technician will decline to apply product over a suspect nail and suggest a pharmacy or clinic visit first. That refusal is not upselling caution; it is the professional protecting you and her licence.