Listen, as an ex-con artist who is in fact a freelancer, just ask. Lots of artists at cons make fanart or make "easy to buy" content because that's what you do in a con, you gotta make specific content if you want people to first notice it and then buy it.
Whenever someone comes and asks for a personal piece, then that means you surpassed those two barriers first, so it is an accomplishment.
I'm from Latinoamerica, Argentina. Most of people that came to my stand were people that didn't even had money to begin with because yeah, Artist Alley is at the back of the event, between the food, commercial stands and the fact that you have to buy tickets for the con first, it just makes sense people come "empty" to our section.
Most of time people ask if we deliver, if we take orders or even if we have a website or email contant. Be it to buy what we offered at the con or to ask for commissions when they get paid again. So to me, personally, is not even an insult if you go ask for mere curiosity, of course, you'll have those that would be suspicious if you don't buy from them first.
It's not uncommon nor weird that we apply or that we get consultations and end up with no clients at all. It's the same as going to a clothing store, the employee may come and ask if they can help with anything and you immediatly say that you're only watching.
If an artist gets offended then it's their own problem, this is also part of selling stuff or a service, they aren't the only one artist offering commissions and most of people consult before deciding on someone, just don't go overboard about the details of the work you want done because that's like providing extra information that it'll be unnecessary if at the end you don't hire the person. Just consult, ask about prices, time a piece takes, what can they do or what not, then tell the person if you either decided or if you're still deciding.