If you have plenty of time — like starting just before bed and ending before the alarm goes off in the morning, the full system will test everything from the standard volume checks, partition maps, S.M.A.R.T., surface scan, memory, video, processor, bluetooth, network, fans, sensors, and I am confident I am missing some.
If you just want to assure yourself of the long term health prospects of your HD or SSD then I would start with S.M.A.R.T., and look at all of the reported values. If any of them are in the red range or if there are multiple in the yellow range I would start looking for a replacement drive. (Personal opinion I would get an SSD even though an HD would be cheaper.)
Regardless of what S.M.A.R.T. reveals, if the Surface Scan (remember that can take a while so don't get impatient) detects new bad data sectors, I would also be in the market for a new SSD. New bad data sectors is an indication the magnetic media is flaking off the platter and will only get worse over time.
Failures or warnings on the other tests will reveal more esoteric, and unfortunately likely more expensive problems with various other hardware systems.
NOTE: Generally testing can be performed on the boot drive, but repairs have to be run from another boot volume and often testing is more accurate when performed from another boot volume. Either the E-Drive or IMHO even better a ProToGo boot volume on a USB 3 Flash Drive will fill that need nicely and ProToGo is now an "in the box" part of TechTool Pro 8 or 9.
