How to know whether the bottle nipple is damaged or not? What are the indications and how frequently should it be replaced?
Most brands recommend replacing the nipples on a regular basis usually 3 months I think, however I've found they tend to last longer than their recommendations. Usually by the time the baby is ready to move to the next flow the nipples are still working fine. It helps to take good care of them and not use brushes to clean them.
As to how to tell if there's damage the best thing to do is compare them to a new, unused one. If there are marks, cuts, or big creases that is a sign that the nipple's pretty worn. Put some hot water in the bottle then turn it over and see how quickly the water comes out compared to the new one. If you have much more water coming out of the used one than the new one then there may be a tear or it's just plain worn.
I too have found that nipples tend to last longer than the recommendations; they just want another $20 from me to replace all the nipples in all my baby's bottles every 3 months. While they do grow into more free-flowing nipples over time, once you reach the Stage 3 and 4 steady-state the nipples are durable enough for your child to use for the 6 to 9 months (or longer) until they've moved on to sippy cups, unless your child is actively teething on them (and our daughter found that we gave her plenty of other more chewy things than the bottle).
I prefer the clear silicone nipples to the yellowish beige rubber, and those are easy to tell when they've been damaged; you'll see the split almost like you would a crack in glass. Typically the cracks show best when dry. Especially check around the tip; for bottles that aren't y-cut, any visible split anywhere in the silicone is an indicator that it's about to fail if it hasn't already, and it needs to be replaced.