You should see krausen- the white foamy "head" on the beer. Or, if fermentation has already occurred, you will see a nasty ring just above the level of the beer. That's a ring left from the krausen. Either one of those signs can tell you if fermentation has taken place or is taking place now. While it's open, use a sanitized turkey baster and take a sample for a SG reading. That's the "for sure" way to know where you're at.