I get asked this question lots - here's how to check your available space step-by-step when running Apple Operating System OSX 10.7 or greater.
Click the Apple icon in the upper left hand corner of your screen and this drop-down menu will appear.
Click About This Mac from the drop-down menu. The following dialog box will appear.
If you are not sure which OS version you are running you can see it here. Notice I'm currently running OS X Version 10.8.4
Click the More Info... button and the following will appear (note: I've covered up my serial number).
Click the Storage (top of dialog box between Displays and Memory) option for the following.
There you have it broken down nicely into Audio, Movies, Photos, Apps and Other categories.
So..... this is a little embarrassing..... I've only got a little over 1.7 GB free..... looks like I need to do a little drive housekeeping or I'm going to be out of space soon :)