This is how I roll with pretty much anyone anyway, 95% of the time.

To answer the question though, I think it's important to just observe what everyone else is doing, and do what they do.

If I'm traveling, and have no idea where I'll be training on my trip, I make sure to pack a white gi, in the rare event that I go to one of "those" gyms.

I also try and reach out to the owner/ instructor, both to make sure their training schedule is correct, but also to make sure they're OK with a surprise black belt showing up to their gym.

Otherwise, I try and be as conservative as possible: bow on/off the mat, wear a rash guard under my gi, introduce myself to the head instructor, etc...

I have stopped into a bunch of different gyms on vacation over the years and while traveling for work. I just always have stopped into the facility to speak with the professor before training. I do this normally a day or so in advance depending on the location.

When I relocated to a new state, I explored suggestions from friends and a ton of people on here gave me great suggestions for my area. Then I did the same, I went in and explained my situation, watched a practice and then I knew if I wanted come back for a class. For me this made it easy to tell if the location was a fit for me or not.

Sometimes also reaching out to the Gym via facebook is a great way to see if it's cool, asking about a mat fee, etc. I always try to call but a lot of schools don't answer their phone. I explain who I am, why I am looking to train there (visiting, vacation etc) and who I train under / association. Most could give a shit, but I have run into folks who don't want "outsiders" at their school which I found / find weird but whatever.

I always ask about a mat fee and if they waive it / say don't worry about it I always buy a patch/ t-shirt /sticker something to support them. Plus I usually grab a sticker anyways for my water bottle.

Pretty much hit it all man. I travel for work and will research gyms in the area and reach out before I go. That way I can introduce myself, figure out any gear requirements (white unpatched Gi is my go to) and ask about mat/drop in fees. Some places have them some dont. Try and roll lowkey and respectful to keep myself and partner safe. Been offered to roll with most of the professors at gyms Ive visited so figure thats a good sign Im not being a spazz. Love to pick up "swag" if they have any (shirt, sticker, etc) and leave them a positive review/ social post. Haven't had to switch my home gym yet.

Contact the gym before dropping in. Arrive early. Introduce myself to at least to the person in charge of the class, if it's a smaller place go shake everybody's hand who are there before the class starts. Observe where you should place your gym bag / water bottle / slippers. I don't care for formalities but I tend to bow when entering the mat when visiting places. Line up as last of your rank if they do that stuff. Also observe whether the coach is being called by their name or "coach" or "professor" and do the same. Be nice.

I tend to wear whatever-colored gi or nogi gear I have, with whatever patches, just needs to be smelling fresh. Never been a problem in the 40+ academies visited not having a boring-ass white uniform. Wear a rashguard under the gi top just in case.

Roll just normally. I'm a lazy person however, so I tend to take it as easy as the partner allows me to.

If it's a good experience (always has been), drop a Google/Fb review after and/or social media post.