The best way to know would be knowing what the electronic configuration of the element is. The electronic configuration of sulfur is 1s2 2p2 2p6 3s2 3p4. The p subshell can house a maximum of 6 electrons, but at this time it clutches 4 electrons, which means that sulfur has 2 unpaired electrons, both placed in the p-subshell. Or it could also be seen easily in the Lewis Dot Structure of Sulfur, you can see that there are two that has no pairs.