How do you check accuracy?

[cbrenthus]

I see alot of threads and post where people say that their watch is gaining or losing seconds. My question is how do you know? What are you comparing the watch against to see how accurate it is?

The only thing I have tried is opening the time/date window on my computer, and comparing second hands. It appears to have gained ~7 seconds in a week, not bad at all from what I understand. Is this a proper way to compare?