S8 is not giant. Neither is ethanol. Both have molecular formulae, which tells you how many atoms there are in the molecule. You cannot know how many atoms there are in giant covalent substances (note I do not call them molecules, although some refer to them as macromolecules, I don't like the term).
The giant moleculars are graphite, diamond, silicon, silicon dioxide, silicon carbide, boron and a few (ish) others. Although RMIM is right, the two forms of carbon are pretty much the only ones you'll come across.
The other thing to look out for is melting/boiling point data (or state of matter). If it is high (thousands of degrees) then it'll be giant. If it is relatively low (up to a few hundred) it'll be simple.