I know of some people who, to make sure seeds will germinate, put the seeds in water to soak for a day or two. This causes some of the seeds to sink and some to float. I know one of those reactions is the reaction of a "good" seed. Do good seeds sink, or do good seeds float? I can't fucking remember. Thanks all.
generally, good seeds sink. But, a lot of the times, the surface tension of the water will float a seed anyway. Give it a poke with your finger and try to sink it. If it still floats, it's probably junk.
Even if the old sink-float thing is true, I would suggest going ahead and trying to germ a floating seed anyway. What do you have to lose, a couple of extra paper towels? All this for a seed that looks viable on the outside, e.g. brown or grey color and possibly mottled with blackish stripes or spots. If it's green or white on the outside don't even bother.