I sure as shit talk to them when they get there, then talk again when they are ready to leave, ask them to walk me through what they did, and double check the invoice for accuracy.

Especially for "subscription" services. I've found it's best to trust, but verify that they are coming out as frequently as you're paying them to and doing everything you're paying them for. It's not even a malice thing; humans make mistakes.

Depends on the job.

I hire you to change the electrical panel and you charge me X to do it, I'll leave you alone and might even leave the house because the electricity is out.

If I pay 150$/h to have something fixed, you can be sure I won't be far.

But if I want to watch them work, I ask first if it's ok for them.