The best usage guide you'll find is in-game.
Use !help.
The !skillcheck command uses the name of any skill, ability, or saving throw without spaces. The DC is optional. e.g.
This is especially handy for when the game is paused.
When DMs use this command, all PCs in the area will roll a check/save. DMs may use the option silent to send the results to the DM channel instead.