I really don't agree with the "when they are interested in what your eating" sign because babies are going to be interested in whatever your doing and mimic you..doesn't actually mean they are ready for solids. They are just learning they are able to grab and everything goes in their mouth lol Here are some signs I got from wholesomebabyfood.com
Loss of tongue-thrust reflex - This allows baby to drink and swallow liquids with ease; with the tongue-thrust reflex still present, baby may simply drink in liquid purees or push the food back out. According to Dr. Jim Sears, in the first four months the tongue thrust reflex protects the infant against choking. When any unusual substance is placed on the tongue, it automatically protrudes outward rather than back. Between four and six months this reflex gradually diminishes, giving the glob of cereal a fighting chance of making it from the tongue to the tummy
Ability to let you know she is full from a "meal" with signs such as turning away from the bottle or breast. This is important so that baby is able to self-regulate the amount of food being eaten. This helps stop baby from accidentally overeating as parents may continue to feed baby thinking that she is still hungry.
Ability to sit up and hold head up unassisted
Interest in your food (we tend to disagree with this one as when a baby reaches the age of 4-6 months, he is interested in putting everything in his mouth!)
Doubling of birth weight
Frequently waking in the middle of the night when a solid sleeping pattern had been established. This may not be the best indicator that your baby is ready for solids. Please keep in mind that a growth spurt will occur between 3-4 months of age, 6-7 months of age and also 9-10 months of age. Baby may also be waking due to an illness or teething.
These are 2 more added in from babycenter.com
Chewing motions. Your baby's mouth and tongue develop in sync with his digestive system. To start solids, he should be able to move food to the back of his mouth and swallow. As he learns to swallow efficiently, you may notice less drooling. He may also be teething around the same time.
• Significant weight gain. Most babies are ready to eat solids when they've doubled their birth weight (or weigh about 15 pounds) and are at least 4 months old.