No real way to tell without germinating. Most seeds are a greyish green w dark spots or stripes. Some are just dark. Squeese the seed between forefinger & thumb to make sure it hasn't cracked already. To germinate just place in a glass of warm water and keep it out of sunlight. Usually within a week it will crack open and the taproot will pop out. Plant the seed without touching the root about a 1/4 inch under soil and within another week your sprout should poke through. Of course you want to wait till next year unless your cultivating indoors. Seeds will keep for a couple of years.
or you could try the damp paper towel in a sandwich bag method. but yea as long as the seeds arent tiny and whitish green (not mature) the seeds should crack unless there just no good.