A good mechanic saves you money over the long run, so it is worth finding one before you have an emergency. The best workshops are busy, explain things clearly and don't pressure you into work you didn't ask for.
Ask whether they use genuine or aftermarket parts, and get the difference priced for you. Ask how long the job will take and whether they will call you before doing anything beyond the quote. A mechanic who is happy to explain the work is one you can build a relationship with.
Start with a small job — a service or a brake check — and judge them on how clearly they report what they found. Once you trust a workshop, stay with it: a consistent service history adds value when you eventually sell the car.