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Note that the big search engines have many data centers and different people may get slightly different results, depending on their geo-location and also their browser software. This kind of "personalization" is relatively new, and will probably become even more common going forward, and potentially can include many other personalized factors.
You can see if the search engines are SPIDERING your forums by checking your server logs -- but this would not mean that your site is in the index. For that you can use a special search query that both Yahoo and Google currently support -- the site: operator (note the colon on the end)
So you can do a search for site:yourdomain.com and the result will show all the pages that engine currently has in their index.
You can also use your server logs to discover what search terms are actually bringing people to your pages -- a very good practice, IMO.