You have two ways:
Method A would be like phflack said. Find the ID of an object (like findclientbyname(name) to get the ID of a client or look at an object and say /getID to get the ID of what you're looking at ingame), open the console and type idnumber.dump(); to get a list of variables and commands you can use on said object.
Methon B would begin writing a command you have some idea of (say you're looking for a command that plays audio and you know it begins with alx) so you type "alx" in your console and press TAB to autocomplete. Press TAB again to cycle through other commands until the end. Shift-TAB cycles backwards.