I work in ladies wear. When a guy comes into the store and appears to be cross dressed (discreetly) is it appropriate when assisting him to ask is this for yourself or to offer him the opportunity to try an item on rather than suggesting he can return it if it does not fit.
Hm. Interesting question. I used to work in women's retail and I had cross-dressing and transgendered clients all the time. I treated them the same way that I would treat any of my other customers: greeted them, offered my assistance, offered to set them up with a fitting room, checked on them to see if they needed anything, and cashed them out---all the while being polite and courteous.
I don't recall having any customers who came in dressed "discretely". My only worry would be that if I made a mistake and thought they were shopping for themselves, when they really weren't into crossdressing and were actually shopping for a woman in their life. I wouldn't want to accidentally offend them. If you're fairly certain that they're shopping for themself, then I would offer them a fitting room. If you're not sure, then hopefully they'll ask to use a fitting room if they want one. I'm not really sure what the "appropriate response" would be.
When I worked at a clothing store we had tape measures/size charts so we could gauge a person's size pretty easily without them trying anything on. If he's already wearing it odds are he knows what size he wants. Just sell him what he wants and don't worry about it.
I wouldn't ask if the item is for him specifically. I would keep things general and ask him if he needs any assistance looking for anything. If he goes into the dressing room then have him mention the size he might need. This will keep it professional and not awkward.
Hope this helps.
I'd imagine it depends on who you're talking to. I would just try to read people, and if they seem like they might be receptive go ahead. Just be discreet and make sure no one else can hear you.
Of course, I called a fat man "ma'am" once, so maybe my advice isn't excellent..