What if you see an ad on your own website that interests you? In the 468x60 banners, they don't give you the urls to the web site advertising on your site. Are you aloud to check it out if it interests you in that situation? Also, what is Google's stance on clicking the ads of other web sites?
The AdSense TOS is written in pretty clear English. Hence, you know the answer to your first question. Because as the good business person you are, you read and understood the TOS you agreed to and which formed the fundamental business contract between you and Google. You can click the ads of other websites if you made that decision to click yourself and were not influenced or incitentivised to do so.
Of course you are NOT allowed to check it on your own site. If you click on it the advertiser would have to pay for the click.
@WebDivx: you already know the answer before you even asked the question. If you really want to check out the ads, use the Google Adsense Preview Tool (for IE only) and note the URLs and then typing them in manually into the browser. Here's the link to it: https://www.google.com/support/adsense/bin/topic.py?topic=160
Dude, don't listen to them. They're just jealous. You should click on your own ads as many times as you want. In fact, I know for a fact that my Google rep encourages me to click on my own ads, if you can believe it. Pinky swear.