OG, yeast, and temp would be helpful. But in general the only way to tell is to check gravity. Wait 3 to 4 days and check again. No change = done. If you don't want to waste beer or have a refractometer, just let it sit for 2 to 3 weeks unless it it a high gravity monster.
Yeah,when the rapid bubbling slows or stops,only initial fermentation is done. It'll now slowly,unevenfully creep down to FG from here. Then 3-7 days for it to clean up any by products of fermentation & settle out clear or slightly misty. Then prime & bottle.