Write site:yoursite.xxx in Google search field or sitewww).yoursite.xxx depending if you use www or not. I cant write urls yet, thats why i use (). That works with google... i also think you can see indexed pages in google webmaster tool.
Go to yahoo site explorer(google for it as i cant use live links yet) and enter your url there. This will give you yahoo's numbers. With Google and Bing I am seeing some changes in recent months as to the correct number. The "site:" command is not as accurate as it used to be for sure. A few things you can use to get a better idea of how well your site is indexed. 1. If the site is large, setup multiple sitemap files for different areas of your site. i.e. if you have a site in multiple languages, say English, Spanish, French and German, create a sitemap for each one and then check Google webmaster tools to find out if any parts of the site are not being indexed well. Same applies if you have an ecommerce site. Can do the same with certain categories etc. 2. If you use Google Analaytics, go into the "content", "landing pages" section and compare this over a number of months. This will tell you the number of landing pages currently being hit. The higher the number the better indexed your sites pages are. 3. When you use the "site:" command on Google use allinurl to check certain areas of the site. For example if you had say a real estate related site and wanted to check how many listings under "villas" you had indexed you may do something like allinurl:/villas/ site:yoursiteurl.com This will give you a more accurate idea of which parts of the site are indexed. Like I say. Google and bing are not providing accurate data at this time, so can't be relied upon 100% Hope this helps.
If you want more details you can also use Google Webmaster Tool. Submit your sitemap and you can get details on how many indexed pages and any crawl errors that may have occurred when Google Bot visit your site.