9 out of 10 .com sites are not designed to be used on a mobile phone. Stop guessing which ones work.
There is only one standard that guarantees you a site specifically designed for your mobile phone and that's .mobi. Just like how sites ending in .gov (such as IRS.gov) guarantee you a legitimate government site, sites ending with the .mobi "trustmark" guarantee you a legitimate mobile site.
Consider it a seal of approval that the site is mobile certified. Why waste time shooting in the dark?