This seems completely ridiculous to me, as do some of the replies. The idea that you don't know whether or not you're getting the employee's 'best' or if the employee is 'slacking off' is frankly your problem at best. More accurately, it's irrelevant.
I think this is one of the ideas that remote work can help improve. I'd like to be paid for the value of my work, not for your impression of the amount of my potential that I appear to be exercising in your favor... and that's what it is, right? If the employee is in the office you determine whether he/she is 'slacking off' by whether you see them browsing facebook (or HN?)... That's a flawed way to evaluate someone's worth as your employee. I find it interesting and compelling that remote employment forces us to change that dynamic.