When the meditation is practiced continuously and when one is able to remain in meditation for a long time, just like when you are able to hold the mind in between eyebrows in a total thoughtlessness state without any imagination of thoughts or visions, at least for one hour, then you can tell that you are meditating. Until then you are trying to meditate. When gradually you practice, practice and practice, this is the mantra. Everyday without fail you practice. As your mind starts receding you would acquire a taste for the meditation. So one day when you are able to sit for eight to ten hours of meditation in one stretch, in one single sitting, like when you are able to hold the mind in between the eyebrows to total thoughtlessness state at least for eight to ten hours at a stretch, in a single sitting, then you will be capable of doing Tapas. That's how the meditation gets converted into Tapas, eventually.