You may be recalling the Starting Hand Simulator, one of the tabs at the top will simulate your opening hands, and allows draws and mulligans as well. I totally understand the difficulty with testing a paper deck, there's no replacement for playtime, but it can be hard to find people in person. So in terms of online play, there are the programs mentioned above, but also mtgo of course, if you have money to get digital versions of the cards as well.
good luck!