How to find out if an installed Eclipse is 32 or 64 bit version?

Hit Ctrl+Alt+Del to open the Windows Task manager and switch to the processes tab.

32-bit programs should be marked with *32.

Open eclipse.ini in the installation directory, and observe the line with text:

plugins/org.eclipse.equinox.launcher.win32.win32.x86_64_1.0.200.v20090519 then it is 64 bit.

If it would be plugins/org.eclipse.equinox.launcher.win32.win32.x86_32_1.0.200.v20090519 then it is 32 bit.

Help -> About Eclipse -> Installation Details -> tab Configuration

Look for -arch, and below it you'll see either x86_64 (meaning 64bit) or x86 (meaning 32bit).