The only guide you have is the Ad Requirements and Guidelines [adwords.google.com] section of their advertisers' guide.
If you don't try to get around their rules, you have a pretty good shot at getting your ads approved.
Remember that the approval process is performed by humans in their editorial department, and there are no programs (AFAIK) that will tell you in advance whether ad text will be approved or not.
One of the key reasons for disapproval, by the way, is not having enough relevant content on the landing page. In other words, if you try to set up an ad for "orange widgets" and the Target URL goes to a page that has nothing to do with "orange widgets", that ad will not be approved, regardless of how well the text meets the guidelines.
