How can I tell if it is a gentle filter?

You didn't say what size the tank is, but a sponge filter can be used for a tank up to 10 gallons.

There are some corner filters you could use too, I have this 1 in a 5 gallon tank http://www.thatpetplace.com/pet/group/10602/produc... It uses suction cups to stick to the tank wall, and unlike most corner filters, the part with the filter pad can be detached so you don't have to take the whole thing out.

And there are power filters you can buy where you can adjust how fast the water comes out. I have 2 of these http://www.thatpetplace.com/pet/group/10463/produc... and 1 of the *mini* version of this http://www.thatpetplace.com/pet/group/10618/produc... which are good for tanks around 5 gallons.

You don't want to get a filter that's too small if you get a power filter, so get 1 that rated for the size of your tank or the next smallest, and make sure the 1 you get has a way to adjust the flow.

You can also use plats in front of where the water comes out to break up the force of the water, or make a filter baffle http://www.petfish.net/articles/Do-It-Yourself/cur... if it's still too strong.

I would highly recommend a sponge filter as well. I have a whisper in-tank filter in my betta 2.5 gallon but if the tank is full of water the filter won't move the surface water and it grows algae on the surface. BAD

So I have to keep an inch or two below the filter so that it falls in and creates surface movement. A sponge filter would eliminate this problem as the bubbles would keep the surface moving all the time without creating too much current

They will not have strong/medium/gentle on the label, you just have to guess. If you are buying a 10 gallon filter for a 4-5 gallon tank which is ideal for a Beta, then my guess is it's going to be stronger than a filter made for 5 gallons. If you buy a filter made for the size of your tank and it's still too strong which I doubt, then you can buy a filter sponge to put on it.

get the filter rated one down from the size of the tank. so instead of a 10 gallon filter for a ten gallon tank, get the next level down, which is probably for 5 gallons.

http://bettacare101.googlepages.com/

A sponge filter creates very little disturbance, and are fairly cheap.

I don't like box set aquariums because I would rather choose products that suit my animals, and buying items separately is usually cheaper.