Good ol' Google defines trust as the 'the firm belief in the reliability, truth, ability, or strength of someone or something.' One way to know if your technician's reliable and truthful is to follow up on the work that they did. I mean, if you want to know if he really replaced your water pump you're going to have to visually check and see if you've got a new water pump. Of course, that seems a little extreme seeing as how you'd need to take the engine apart. Stay with me here... why not follow up with them on something small? Like an oil change or tire change? I'm a firm believer that if you can trust me with small jobs, you can trust me with big repairs. If I can be faithful with an oil change, I will be faithful with an engine replacement. It's a matter of integrity.
After you've confirmed that the technician checked the tire pressures, take your tire pressure gauge and check them yourself. They should all be topped up to the correct pressure. Got clear nail polish? Put some on the tire pressure valve caps at the seal. Check after you get the car back to see if the seal has been broken. The seal has to be broken if they took the caps off to check the tire pressure.
If you've asked the technician to check and top-up fluid levels, check your fluid levels and make sure they're at the correct level when you get the car back. For a neat tip about those not-so-easy-to-read fluid reservoirs, watch our webisode.