Where are you located ?
I guese the best i can offer is that the seeds should generally be free of any pod material , clean mould free & look fresh not several year old faded colours . I woild myself prefer the seeds to have an individual labels statingthe type of variety , the date it was harvested and th date it was packed because fromthere you can work out the likley viability period that they will still germinate at over 70 % per sowing.
thre are some briliant online charts that show all manner of seed viability listing , also the optimum germination temps givingnthe 5 of seed likely to germinate at that temp.
What ever you buy perhaps sow 10 of each type of seeds on or between damp kitchen roll squares laid on a Pyrex lid and place the dish over the top to make the germination chamber. Multiply the number of germination's by ten to get your percentages .. if the seed are quality this % rate of germination should be close to the germination %'s suggested in the on line lists.