Over the years, through my experience and reading the experiences of others. The 'euphoria' (I use that term loosely with kava) felt can differ with different people and their own body chemistry. Ever since I started, the headier/kavain kavain strains were always the most difficult for me to achieve noticeable effects from, although they are often regarded as the most enjoyable varieties to most. Another thing happens, to some people who aren't completely out of the priming stage (RT), they describe feeling euphoric for a short burst followed by just about nothing. For me, the only time I reach something that could be described as euphoric, is when I've had a good strong batch and full-on kava rush sweeps over me. These experiences are few and far between for me, unfortunately, but the peak experience lasts at least 45-60min when it happens.
In a different way, I feel a very mild euphoria after my 1st or 2nd shell of any kava...because the feeling of my mind and body finally slowing down & relaxing is a welcome relief.