If the command or feature won't destroy anything when it doesn't work, try it! For instance, don't experiment with rm, the command that removes files. But cat, a command that shows files, probably won't hurt anything if some feature doesn't work with your version.

Look at the online manual or check your vendor's latest printed manuals. However, even these can be wrong. For instance, your system administrator may have installed a local version of a command that works differently -- but not updated the online documentation. Be careful with "generic" manuals, the kind you buy at a bookstore; there are lots of versions of Unix, and the manual may not match your version closely enough.

Ask your system administrator or another "guru" for help before you use a command that might be dangerous.