
Right Click on the icon and select "Show Package Contents" now go to Content/MacOS/, here you would see eclipse.ini file, lets see what details this file contains,

-startup ../../../plugins/org.eclipse.equinox.launcher_1.1.0.v20100507.jar --launcher.library ../../../plugins/org.eclipse.equinox.launcher.cocoa.macosx.x86_64_1.1.0.v20100503 -product org.eclipse.epp.package.php.product --launcher.defaultAction openFile -showsplash org.eclipse.platform --launcher.XXMaxPermSize 256m --launcher.defaultAction openFile -vmargs -Dosgi.requiredJavaVersion=1.5 -XstartOnFirstThread -Dorg.eclipse.swt.internal.carbon.smallFonts -XX:MaxPermSize=256m -Xms40m -Xmx384m -Xdock:icon=../Resources/Eclipse.icns -XstartOnFirstThread -Dorg.eclipse.swt.internal.carbon.smallFonts
Check the 4rd line,
../../../plugins/org.eclipse.equinox.launcher.cocoa.macosx.x86_64_1.1.0.v20100503
If it contains x86_64 then your eclipse is 64-bit or else if it contains just x86 then it is a 32-bit setup.