How can I tell if my instructor is 'a good one'?

There are two simple ways to check this.

Ask around friends and family to see if they can recommend someone.
You may already know people who have been taught by an instructor you are considering, if so ask them what they thought.
Word of mouth recommendation is generally the most trustworthy.

Check the instructors website to see if there is any student feedback.
- mine is here

Once a driving instructor is fully qualified (ie they have passed the third stage test and been awarded a GREEN badge) they are periodically assesed by the driving standards agency.

Qualified instructors are graded during these regular 'check tests' performed by the Driving Standards Agency (possibly during one of your lessons!)

The grading scale is from grade 1 to grade 6 (with 6 being the highest).
Don't be afraid to ask your instructor about their grading.