Serious musicians often experience their instruments as an extension of their body. In any case, their instruments are usually deeply personal things.
High-level instruments also tend to be very expensive and difficult to replace.
Therefore, you should never play a musician’s instrument without asking first.
It’s a good idea to err on the side of not asking, unless you have a very good reason to suspect that they might be ok with sharing their instrument. (Eg: you’re very close friends and you’re both musicians, or you know they’ve been ok with other people playing them sometimes). And it’s good to ask in a way that makes it clear that it’s a request, not a demand.
Pianos and keyboards are a partial exception - since it’s relatively difficult to break them, and they’re usually played by more than one person, most people who have pianos are willing to let other people play them. But it’s still good form to ask.
A few points to add.
Everything said above can apply to items used in other kinds of art/craft, though to varying degrees. Craft objects are part of a culture where sharing/collaborating/looking at each other’s tools is more common, but permission must be received beforehand, and these tools need to be treated with care.
If a person’s tools/instrument (including computers) are part of how they earn a living, it is probably best not to touch/use their tools unless you can afford to replace them. An exception is if the owner offers to teach you how to use their tools, or is closely supervising you.
People have tried to use my fabric scissors on paper and that is so rude.
Basically everything considered “office supplies” in my space is really part of my art practice and should be treated as something to not play around with. Gosh, some of my pens are for certain things only. Artists, makers of any kind, really need to have that stuff respected.
Please oh please never assume you can use my sewing machine.
Seconding all the art stuff. I very readily give permission, but I need to know you’re using it and may want to supervise while you do.
And if you put any marking instrument near my artwork, I may not forgive you, even if it’s a joke.
Yes. Threatening to destroy someone else’s work is mean. It’s not a funny joke. Especially if you put yourself in a position in which it would be easy to accidentally destroy it.