Yep, that's pretty much it. Put maybe a cup of beer into a clean container. Add a crapload (technical term) of yeast....ANY yeast, even bread yeast will work. You want to see if there's any possible way it will ferment further. If it does, you know you have fermentables left and you can pitch more yeast into your fermenter. If it doesn't, all the fermentables are used up and adding more yeast won't help. FWIW, I think that's what's going on with you.