Your third eye is your pineal gland. It has the same type of cones and rods that your eye balls have, that is why they call it the third eye. How you can tell if it is open is by picturing something in your *mind's eye* for instance, on a beach looking out, and in the sky is a rainbow. Can you picture that scene clearly?
The pineal gland can calcify for many reasons, such as but not limited to; diet and flouride tooth paste and flouride in drinking water. This is something you don't want to happen.
You can de-calcify your pineal gland easily.
I would suggest looking up Hibiscus Moon on yourtube, go to her channel and she has several video's on the topic. She has a master's degree in science and was head of chair for 10 years. I trust in her knowledge and expertise.
On a personal note, having a clear pineal gland and learning to work with your mind's eye is totally different than, for instance, have your chakra's open, palm chkra's open. Some people feel that being so OPEN makes you vunerable to negative energy. That is true if you do not understand how it works to deflect that type of energy.
What you think about, you bring about.
I would also suggest learning about working with energy, and reading books like, The Hands of Light and Light Emerging. Learn Chakra's and how to work with them, and from there graduate with pineal gland. Advanced people can work with Kundulini, only I wouldn't suggest that if you aren't at least under a master.
Just like a staircase, you have to climb it to reach the top. Make sure you have strong footing (that would be your knowledge). Play safe, be aware and have some fun with the science behind it.