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Jhana will be absolutely unmistakable from your prior meditations. If it's jhana, you'll be talking about your fortunate achievement for a long, long time and you will be so very amazed at what your mind was able to do! (When teachers interview students to help them distinguish a very deep meditation from the still deeper jhana, a first criteria they look for is the student's excitement level - when it's jhana, the student can't stop talking about it!)
A.Brahm's online description and DIY guide was exactly right for me. He helped me to fall into the first jhana, so perhaps his instructions will speak to you, too! I won't detail my personal jhana experience (because your mileage may vary, just as your mind's imagery or lack of imagery will vary). So here I'll put out just the barest information that defines the common experience of the jhanas.
Timelessness is experienced in all of the jhanas. This is because no clues remain to signal time's passage. So time ceases to have meaning. To get this deep in the first place we had to first stop stringing words into sentences in our head near the start of our meditation. And with that inner dialog gone, that's one less clue that time continues to pass. Just prior to falling into jhana, hearing & sight & smell & taste have long since been shut down. So any external input from these sources aren't marking time any longer, either. It's only your breath-object marking time, and your breath is by now so light that you don't hear it; you're detecting it through touch-sense only. Your breath-object (or whatever other meditation-object you're using) has served you well to help get you to this point, but now is when you let even that go in order to fall into the first jhana. And when you do finally lose track of the breath,
HANG ONTO YOUR HATS !!!! WHOA !!!
Since your breath-object was the final thing marking time, you instantly gain the insight of what it would be like to live in a timeless realm since you are indeed in a timeless realm. (This is why your sitting time will likely be longer than usual too - my jhana experience was a 2hr sit whereas my usual sit is 1hr). This experience of no-time, coupled with that of the mind having detached from the burden of the other 5 senses, can be likened to having dropped a 100lb backpack you've been lugging around your whole life. This is the minimum shared common experience, and you will likely experience even more, as I did. And the profound liberty you feel after being freed from the 5 senses will give you the insight of knowing how deeply we all suffer just because of those senses. It is truly amazing! So you know you've experienced jhana if there was timelessness and liberation -- and it is markedly different from anything you've experienced before -- if your previous meditations are like a trip to Mars, then jhana takes to you to the outer gas giants!
Please don't confuse jhana with the one-pointedness of breath that occurs just prior to jhana (although this latter is a fantastic place to dwell too). It's a high-energy blissful state where you're so focused upon the breath that sight + smell + hearing + taste have shut down; only the touch-sense remains, which is marking time with the breath. A.Brahm calls this stage the "beautiful breath" and you need to generate tons of "piti" to get this deep. A.Brahm's procedure for elevating piti ever higher and higher using a sort of contentment-feedback-loop is what helped me to achieve this one-pointed state, and then fall into first jhana.
If you are at all interested in these deeper levels of meditation I do whole-heartedly recommend you search "ajahn brahm jhana pdf" to learn more!