When we visited Kara's pediatrician when she was about 6 months old to consult about feeding, Doctora explained thoroughly and gave us a list of do's and dont's when introducing solids (this is why I love her! She will really spend time to explain important points to you). Sharing everything I've learned below (not just from our pedia but also from more than a year of doing BLW and teaching other parents how to).
How to know if your baby is ready for solids:
Baby is at least 6 months old (WHO and American Academy of Pediatrics recommend exclusive breastfeeding for the first 6 months of a baby's life)
Baby can sit up well without support
Baby doesn't have the tongue-thrust reflex anymore (as in they automatically push solids out of their mouths to prevent choking - they usually outgrow this at 4 months, but some do much later)
Baby must be willing to chew or looks interested to eat
Baby is starting to pick things up with his thumb and forefinger (Pincer grasp)