Most questions about fermentation can be answered by using your hydrometer. You get the most information from the hydrometer by comparing two readings so always take a reading when you first mix up the brew – this is your baseline reading.
After adding the yeast, fermentation will typically take about 6 to
18 hours to start. During this time nothing will appear to be happening.
The yeast is preparing to ferment by reproducing, and making the necessary enzymes. During this period the yeast use oxygen.,
The first signs of fermentation will be wisps of foam starting to form on the surface of the wort. These will get bigger and eventually join up to form a layer of foam on top of the beer. The airlock should start to bubble.
If the airlock has not started to bubble within 24 hours do not panic. If there is froth and foam on top of the beer then it is fermenting. All it means is that the fermenter is not properly sealed. This is nothing to worry about, there is enough of a physical barrier to protect the beer. People will often over-tighten the lid, and the rubber lid seal can catch on imperfections on the rim of the fermenter and lift to create a leak. Undo the fermenter lid half a turn and then do it up one quarter of a turn.
If it has been some time since you pitched the yeast and fermentation appears not to have started then take a hydrometer reading. If the reading is a lot less than your initial reading then fermentation is probably close to finished: the beer has fermented quickly and you haven’t noticed. This is very common in hot weather. You will probably also see a brown tide mark around the fermenter a centimetre or so above the beer. This is a sure sign of fermentation.
If the hydrometer reading is the same then fermentation has not
started. You may have killed the yeast by adding it when the wort was
too hot, or the yeast may have been old and unhealthy. Dried yeast has a
shelf life of 18 to 24 months and needs to be stored in a cool place. Alternatively, conditions are too cold for the yeast. Most tinned concentrates are supplied with ale yeasts, including those labelled as lagers. Most of these yeasts have difficulty fermenting below 16 to 18 degrees.
If fermentation has definitely not started after several days - ie no change in hydrometer readings, then taste a sample of the wort. If it tastes OK then add another sachet of yeast. It is a good idea to always have a couple of spare sachets of yeast on hand. If the weather is cold then use a lager yeast.
Movement in the airlock is the least reliable indicator of fermentation. Always check with a hydrometer as well as observing and tasting the beer.