Gerry's in Nottingham have their maggots on display, so it's easy to see what's best. I have also tried asking when delivery takes place, but was told this can vary. I used to go to a maggot farm in Newark when I match fished a lot and 'buddied-up' with a fellow angler to make the trip worth while. The smell was something else, but I remember the dominant one was ammonia. Preparing the maggots - riddling, adding maize leal etc was all aprt of the fun, and you could be confident of arriving on the bank with top quality bait. I remember Ivan Marks stressing the importance of having the best bait on the bank, and I wouldn't argue with that!
I really hate paying top dollar for what can be shocking bait, and now that I don't do as much fishing or use as much bait, getting fobbed off with small tough maggots became a recurrent thing. For shops that don't display, I adopt the following:
Look the proprietor straight in the eye, and say clearly and with the suggested emphasis, "My good man, give me X pints of your LARGEST, SOFTEST grubs!". I am not too bothered about colour any more, as I tend to pleasure fish, so when I get asked "What colour?", I simply reply with "Whichever are the LARGEST and SOFTEST". It certainly gets your point across! This works for me, but may not if you are particular about red/bronze/yellow/white etc.