Feeling the chakras (and implicitly the third eye) are very discreet feelings and very different to physical or "solid" feelings (if you touch your hand for example etc). You need to be relaxed enough and part of feeling a chakra is that you "create" so to say this feeling yourself by moving your attention and your vibration or breathing (depends what exercise you do) onto the specific area of the body where the chakra is located. If you have indeed stimulated a said chakra by meditation, you will be able to tell right away because that sensation will persist after you finish the meditation.
Also if this helps, the third eye and any other senses don't "open right away". They need to be developed and practiced on permanently. It is a gradual process, the results are also gradual.