How can you tell if your hair is thinning?

If you are losing a lot of hair in a day, and the hair isn't growing back, that's usually a good sign it's from Androgens.

Thinning shears make hair uneven, unfortunately. Your barber should use his/her scissors to add textture, depending how good/expensive they are.

Really good barbers don't use clippers, believe it or not. Some use scissors instead of clippers, but those barbers are usually more
expensive.

If this is the first time you're noticing it and it's also the first time you've had something different done to your hair, then chances are it's the haircut itself that's making it look this way. As pointed out above, thinning shears are essentially cutting about half the hairs on top of your head significantly shorter than the other half, which makes the hair less "puffy". I used to have barbers do this to mine, and personally I don't think it's the best way to get a hair cut.

Let it grow back out and either go to a different barber, or specifically request that this barber doesn't do this next time and see if it makes a difference.

As for other ways to tell if your hair is thinning, it really just comes down to being able to accurately monitor your hair fall and regrowth. We lose about 100 hairs a day naturally anyway, and these should grow back. If you're losing more than this and they aren't growing back at an equal rate, then you should start looking into some solutions. It's one of those things that "you know when you know". If it's something you're worried about, be vigilant about keeping an eye on it - but at the same time, don't let yourself stress and obsess over it. Remember, generally speaking, hair fall is completely normal.
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I am a patient advocate for Dr. Parsa Mohebi in Los Angeles, CA. My views/opinions are my own and don't necessarily reflect the opinions of Dr. Mohebi and his staff.

This is a great question. In my personal experience, I would not over think it. Hair is subject to a lot of variability in density, thickness, etc. One's diet, stress levels, hormonal levels, etc. are all key variables when it comes to hair looking it is or is not thinning.