How do you know if the clinic/technician is good or bad at doing SMP? What questions should I ask to evaluate them?

The best way to see if your doctor or clinic is good at SMP is to ask them to meet their former patients.

Meeting real scalp pigmentation patients is a great way of checking on the quality and work of a SMP technician or Doctor. Especially if they have a similar skin complexion to you. As this can change the pigment application. Also, the hairline design as this can alter greatly. A natural design is soft and appropriate to your bone structure. You can ask a technician to create a design for you at a consultation. See their artistic ability and ask why they positioned the hairline where they did. 

You are correct to worry about your selection of an SMP facility as many do terrible work. Ask to meet patients, one-on-one and see for yourself if the standard you see meets your standard.  This is critical