I usually compare a whole lot of YT videos of the same model uke, and see if I can find it on Vimeo which is usually better. Now with HD available it's a little easier to hear through the compression etc. I also use headphones, so I can hear even better. Of the ones I have heard and bought, I have overall been happy with them, and they have had the same characteristics I heard online in person, only of course better.
If that's the only way you can really experience them, and you can't buy used from private parties via Craigslist or the like where you can try it first, then pick one and consider it more like renting. If you don't like the instrument after a few months, get another and sell the first. You won't lose that much. A shop local to me rents for $50 a month, then if you buy you get the rent paid deducted, if not it's gone. They only rent the uke or guitar for three months, then you have to decide.