The only way to be certain is to use a complete electro-static-discharge (ESD) setup, including a mat connected to a ground wire, and a grounding strap that you wear around your wrist. Just the grounding strap by itself is mostly useless without the grounded mat.
That said you can take a number of precautions, and you should be OK.
First of all you want to work on a desk that is on some sort of hard surface - not carpeting. Carpet makes is real easy for you to build up static. The amount of static electric charge that is considered dangerous to exposed electronic components is very small. You won't even feel or see a spark. I personally don't like to wear shoes or socks at all when working on electronic equipment. Bare feet are a better conductor, meaning hopefully the static electricity will discharge harmlessly through my feet - and not the components I'm handling. Don't move around a lot either. Shuffling around on the floor, or even your shirt sleeves rubbing against your shirt all generate static electricity.
Second of all, keep all components inside their bags until you absolutely need them. The bags are made from a special material that protects the components from static electricity.
Third, ground yourself. Often. The best way to do this is to touch an exposed drain pipe, or, plug in the power supply of your computer and touch the outer shell. The power supply's 3rd prong is for grounding so any excess static electricity will drain from you that way. The power supply doesn't need to be powered on, just plugged into a properly grounded outlet. DO unplug the power supply when you go to mount it inside the case and hook up all the power cables
Fourth, if you need to remove a component for some reason, put it back inside one of the anti-ESD bags, don't just lie it on top. It will be better protected inside the bag.