Wear the dogi you'd normally wear, and your grade belt with it.
Hmm I suppose people are not familiar with the concept of guest attendances...
If you were going to a club to join and learn the syllabus etc then you might normally expect to start learning from the beginning. But if you were to just visit a club somewhere for whatever reason then you would not normally have that expectation - going to train as maybe a once-off rather than as a regular thing.
Some would still expect you to wear a white belt as you'd not be graded in that system, but many more don't bother with it if you're not coming on a regular basis. They also will not plan around what you may or may not know. For you it can be very much a case of sink-or-swim for that class.
It's quite obvious to regular students that you're a newcomer, and if you're wearing a grade belt they'd most likely assume that you are graded somewhere, and even ask where you're from, what club, etc.
Though it always pays to check with the instructor just to see what their expectation is when you're unsure.

The easiest way is, if in doubt - ask.
Not to confuse the issue, but... Sometimes though you might just want to sneak in a class and try it out - see what it's like - and therefore find a class and turn up with a white belt just to train and check it out. The only trap is that some clubs do actually make you do a grading to get a white belt!


Mick