You can check the freshness of the mix from information given in the packet, or by testing how well the seeds will sprout - fresh seeds sprout easily within a few days after being put in moist dirt, old ones do not. With lab blocks you should check their best before date (which the pet shop keepers have to know - if not take your business elsewhere). Also, find out where the blocks (as well as seed mixes) have been stored. They should be kept in low temperature and dry. If you find insects or maggots in the seeds or lab blocks, do complain to the shop and demand your money back! For some strange reason many pet shop keepers think that the common rules of good business (not selling inferior goods and giving refund if they do, for example) do not apply to them. This is possible because too many people let the pet shops to get away with selling spoiled food (and sick and / or pregnant animals!). You are their customer and without customers they don't have a business at all. Little pet shop rant here: do not buy anything from a shop where animals are not treated well. It is highly arguable if responsible breeders of rodents would ever sell their babies to pet shops either - of course the quality of the pet shop matters here.
The lab blocks are a good alternative, as they contain everything the mouse needs. Furthermore, a picky mouse is not able to pick and choose only those parts of the diet it happens to like. There are usually two different kinds of blocks available, one for growing mice, another one for grown ups.
The basic diet should be given twice a day, especially with larger groups of mice and there should always be some left by the time of feeding
I feed my mice on the parrot mixes found in Coles, Franklins or any pet shop. My mice seem to like eating this.