when i first arrived in florida, i tried to tip the elderly people (or younger ones) at Publix, but they said they would be fired.
so i do what the other mom says -- get to know them if they seem talkative or thank them profusely. most of them are pretty talkative & i've been known to stand in the parking lot talking for a while while they told me about their life experiences......also if it is very very hot, i'll respond -- "you don't want to go outside in this heat, do you?"
usually I can tell if they want a break outside or not by their body language -- if they are already pushing my cart & headed out.....or if they look reluctant and stay standing close to the counter, i say "don't worry about it -- too hot for you outside."
i think sometimes they are bored just standing inside next to the counter & want to go for a walk outside.
Oh, and i always help unload because i'm sure bending & such isn't good for the older workers.