Here are a few signs to look for:

• The starter has a pleasant, sour aroma. This is a good indication that the wild yeast and bacteria in the starter are active and working to ferment the flour.
• The starter has a smooth, cohesive texture. It should be thick and not too watery, with a consistency similar to pancake batter.
• You can hear a 'crack' sound in the bubbles when you're mixing and it doubles in height a few hours after feeding. 
• The starter has increased in size and has visible bubbles on the surface. This is a sign that the wild yeast in the starter is producing carbon dioxide, which is necessary for leavening the bread.
• The starter passes the "float test." Fill a small glass or bowl with water and gently place a spoonful of starter in the water. If the starter floats, it's ready to use for baking. If it sinks, it needs more time to ferment and mature.

Keep in mind that the exact timing of when your starter will be ready to use for baking can vary depending on a number of factors, including the type of flour you're using, the temperature and humidity of your kitchen, and the activity of the wild yeast and bacteria in the starter.

No, you cannot add store-bought yeast to your sourdough starter. A sourdough starter relies on naturally occurring yeasts and bacteria to ferment the dough, rather than commercial yeast.

If your sourdough starter is not becoming active and bubbly, there are a few things you can try:

• Make sure you are using a clean, dry jar and utensils.
• Check the temperature of your environment. A starter will ferment more quickly in a warm, humid environment. A handy tip is to store it in the back of your oven with the light on.
• Make sure you are feeding the starter regularly with equal parts flour and water.
• Check the expiration date of the flour you are using. Old or spoiled flour can inhibit the growth of yeast and bacteria.

You can use tap water to rehydrate your sourdough starter, however for best results it is important to use clean, fresh, filtered water that has not been treated with chlorine or other chemicals.

When you are not using your starter regularly, you can store it in the refrigerator to slow down the fermentation process. When you are ready to bake again, be sure to "activate" it by removing it from the refrigerator and feeding it as usual at least 1 day prior to using again. 

Mould is caused by the receptacle the starter is kept in, or in some cases bacteria in the flour. While sourdough starters are generally indestructible to everything else, we recommend discarding and not moving forward with it if you’re seeing mould.