Ask at the moment of maximum gratitude
The best time to ask is right after you've solved the customer's problem and they're visibly relieved. Don't wait until the invoice is paid and the moment is gone. Train every tech to ask on the spot.
Make it one tap
Create your Google review short link and put it in a text you send before you leave the driveway. The fewer steps between 'happy customer' and 'posted review', the more reviews you get.
Respond to all of them
Thank the good ones by name. Respond to the rough ones calmly and professionally - future customers read how you handle problems. Google rewards businesses that engage.