A lot of clients will see their hair thinning out and begin to wonder if their extensions are the ones doing the damage. I always like to tell my clients - before we jump to conclusions let's analyze your hair.
There are only really 2 ways you can tell if your extensions are damaging your hair:
1) If there is bald spots on the scalp then yes the extensions were too tight/heavy and caused hair fall over time.
2) If there is breakage at the point of attachment (ie: bead, bond, tape) then yes the application must have not be properly done and it cause the hair to snap.
Other ways you can identify what is causing hair thinning...
If there the hair has been bleached and there is breakage where the bleach application was applied (usually root area or midshaft to ends).
You can also tell when you brush through the hair, if it is dry and damaged pieces will just break off into the brush or the hair strands will sort of stick together when brushing through them.
Heat damage is very similar to bleach damage. It will have the same effects and normally is more common on the top layer of hair and around the face as that tends to be the places people will pass over the most with a hot tool.
If the specs of hair follicles get less and less when looking at the scalp area (ie: imagine you are looking at a white piece of paper and it has a ton of little black dots everywhere. Then over time the dots periodically get less and less - not necessary in a particular area). Maybe thinning around the hair line or a ton of baby hairs. This is normally caused by medical reasons: hormones, trauma to body, lack of nutrition, taking certain medications like steroids, ect.
Before you think you have to throw out your hair extensions or stop using them completely because your hair is thinning, analyze your hair - it could be another factor.