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This is something that probably wasn’t given much thought of by most web developers until fairly recently, if it makes you feel any better. The best way would be to actually view the site on the various mobile devices, and that includes tablets and some gaming devices, not just smartphones. Realistically, you, your friends and family are going to have some, but not all of the various devices around for you to use for a test. There are some free online tools that you can use instead of turning everyone you know into a test mule though. The mobiReady Page Test http://ready.mobi/launch.jsp?locale=en_EN checks to see how well your site does according to the W3C Mobile Web Best Practices, and breaks it down as to a number of pass/fail/warnings for a number of design and development issues. There is an Android emulator called Android SDK at http://developer.android.com/sdk/index.html. You have to download it and install it on your computer, then you can see what visitors using an Android device will see when visiting your site.
Also, keep in mind Flash doesn’t work on iPhones and iPads. Good luck!