Determining Ad Inventory

How is this done?

[shrimp]

Other than signing up to be an Adsense advertiser,how can one determine what the actual ad inventory is for a given set of keywords?
I have a page that was my best earner for a long time, I ran 3 sets of horizonal ads on that page. All of a sudden the page only showed one bar of 4 ads.

My page ranks very well in SERPS.

I have tried using different ad formats to see if it is a problem with the page, but still just get one block of ads. I find it very hard to believe that for this particular subject the inventory would drop so suddenly and stay low.

I do not know of any other pages on this topic with multiple Adsense ads to compare with.

I know I can look at Overture or other means to get clues on cost of ads and so forth, but is there any way to determine what Google's ad inventory is at any given time?

[amznVibe]

Search for adsense sandbox tools.

[shrimp]

THANKS!

With so many ads show as avaliable, any clues why I am only getting one banner?

I have many pages laid out in same manner and they show all ads. On this one problem page, I have tried moving my code and switching from banners to towers and combinations of various formats. Waiting several days. Reading forums. Still same thing happens, the first one in code fills , others do not.
Thanks