I assume you are aware of the Shareware tools (Like the five mentioned in this article).
In any case, since you want the free option, you will have to rely in your common sense and the tools you have (this is also true for the Shareware options, which really donxe2x80x99t seem to add much at this stage). Most of the benefit of TechTool Pro and DiskWarrior is to have am emergency plan in place in case of hard drive failure. But that is to be done before the problem :)
Back to your case, I suggest you give Onyx a try to check the status of your HDDxe2x80x99s Smart status. Assuming you donxe2x80x99t hear strange noises in the drive, performing a full Clone to an external drive using Carbon Copy Cloner would provide you with an important piece of information: the computer can read your entire drive, block by block.
Both Onyx and OS X can verify your volume so I suggest you also perform a verify (using Disk Utility for example) and verify your permissions.
Finally, use AppleJack to execute some of the above tests to make sure that the OS is in Single User.
If after performing all of the above, you see nothing xe2x80x9cout of the ordinaryxe2x80x9d, you donxe2x80x99t hear xe2x80x9cstrange click noisesxe2x80x9d or you donxe2x80x99t detect any strange slowness in your drive, you can assume that the drive has not suffered any damage.
But, all things said, Ixe2x80x99d keep my backups up2date just in case. (You do have backups, donxe2x80x99t you?)
Ixe2x80x99ve had drives fall from a desktop to the floor and survive for years without any problems, and some drives tipped 0.5 inches and instantly died. Hard drives are like unicorns. :)