how do you ...

I have a friend w/ a bunch of kids .. lol. we watch each other's kids now & then. I've taken 2 of her boys in my car 1x. At the time I did take them I installed my own seats in my car. When I picked up the boys I told her she was free to check the installs .... she said no, she trusted me (I think she got the idea tha I'm a bit of a carseat freak by me asking for the kids weights/heights before I picked them up ... and maybe a few comments in the past). But -- I won't let anyone else install seats my kids are riding in. So ... how do you explain/expect other people to trust you to transport their kids ... when you don't trust them to transport yours - -without sounding high and mighty (this is if you are not a Tech, because then you just explain that your a tech and why you can explain their seats need fixed).

ETA -- I trust them to transport .. just not to properly install/use carseats

I don't let people transport my kids unless I install the seats myself or I know that they are capable of doing it correctly each and every time.

I'm the same way. I don't let my kids go in others cars unless I install the seats or they can demonstrate to me that the seats can be installed correctly in their car. I have 2 sitters, one is my mom (who keeps seats installed in her car, so I don't worry about it) and the other is my best friend, who I had to teach, even though she has 3 kids of her own (out of seats). She didn't think she could do it, but once I showed her how to use LATCH on her AO, she did it in less than 5 min. She was excited..lol.
That's a hard thing though with your situation. I have a feeling I'd be in the same boat with a friend/neighbor of mine, cause she doesn't use car seats that well. But she knows how paranoid I am about it, so she'd understand.
I would just tell her that you are uncomfortable with your kids riding in the car with anyone whom you have not installed seats for. Even though you aren't a tech, you could say you've done alot of research and know even the most well-meaning parents make mistakes (stats prove it) so you just feel better about it. And it's not her, even the kids grandparents can't take them (even if they can..lol)