I agree with @Apfelsaft above.
Freelancers are service providers. If you are getting the service, and you are happy with the bill, then look to other areas of your business that you can improve on.
If you are getting the service and not happy with the bill, try find compromise or else throw them overboard and replace them.
Let's say you do nit pick how they spend their time - will it include bathroom breaks? What about they deviate from business to a quick joke over the phone with one of the clients. Is that chargeable? What about you are late delivering work to them, do they get to charge while they wait on you even though they have not performed anything functional for your benefit?
Freelancers are not hire cars. Freelancers are like prostitutes. You pay them for a trick, they don't get sick, health or holiday benefits. If they deliver, they eat and live. If they don't deliver, they don't eat, they don't live. I've been in the business since 1994, been hired, and done the hiring so I like to think I know what I am talking about.
I suggest you concentrate on growing your business. Monitor and measure the work you get, by all means, but if you spend too much time monitoring your service suppliers, the business will eventually review your contribution to the bottom line and implement their own cost savings.