How do I tell whether Rosetta is actually installed, and is there any reason not to install it? Rosetta

There is no application to look for because it is a system component, but it does leave something on disk. The check most people use:

/usr/bin/pgrep -q oahd && echo "installed" || echo "not installed"

oahd is the translation daemon, and it runs when Rosetta is present. The other common check is whether /Library/Apple/usr/share/rosetta exists.

To install it deliberately rather than through a prompt:

softwareupdate --install-rosetta --agree-to-license

Worth knowing that the click-through prompt and that command install exactly the same thing, so if you clicked the prompt, you have it. There is no partial state to worry about, which is the reassuring answer to what state your machine is in.

Question 2: it changes nothing until something needs it, and that is by design.

Rosetta translates an Intel binary the first time it is launched, caches the result, and gets out of the way. A native ARM application never touches it. There is no background cost, no memory held, no daemon doing work when nothing translated is running.

So installing it is not a performance decision. The only thing it changes is whether an Intel binary runs or produces "bad CPU type in executable".

Which makes the practical answer to 3 fairly clear: there is no technical downside. The reason people hold out is the one you gave — you want to find out when something is not native, rather than have it silently work. That is a legitimate preference and it is about information, not about the machine.

One thing worth knowing before deciding: Rosetta translates userspace applications, and there are categories it does not cover. Kernel extensions and virtual machines running x86 operating systems are not in scope.

So "I installed Rosetta" does not mean everything Intel will now run — it means Intel applications will. If your reason for holding out was to discover which of your tools have no ARM future, the ones that will still fail after installing it are exactly the ones worth knowing about, and installing it does not hide them.

Building on that: if you want the information without the friction, keep Rosetta installed and check what you are actually running instead.

Activity Monitor has a Kind column showing Apple or Intel per process. Add it once and you can see at a glance what on your machine is still translated. Get Info on any app shows the same thing.

That gets you the diagnostic value you were after without a week of things randomly refusing to launch. Holding out is a slow, painful audit; the Kind column is the same audit in ten seconds.

And it catches the case that matters most, which is a translated process you did not know about — usually a terminal or an editor, quietly making everything you build inside it Intel.