I use this quick-tip frequently to determine if I'm a local administrator. The trouble with some tips is that you just know them, they're so simple so you don't bother to tell people as you're under the assumption they "must have known that". This exact circumstance happened today in the office when a colleague was having problems installing some software. They thought they had local admin rights, but didn't.
Click 1: Right-Click My Computer
Click 2: Select Properties
Click 3: Select the "Computer Name" tab
If the "Change" box is available, you're a local administrator. If it's greyed out, you're not. It's that simple.
So as to combine one tip into two, if you didn't know, you could change Clicks 1 and 2 above with 2 keyboard hits: Windows Key + Break.