You have just noticed that an unknown executable file has been downloaded to your machine.
Is there a way to tell what the file does before opening it?
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What you are looking for is an approach called a "sandbox". It opens or runs the file in a safe bubble to see what it does and to look for malicious actions it might take.
There are many ways to do this and many tools to use. Virtual Machines can be used as a sandbox, there are online sandboxes, and there are tools you can install on your machine that can give you a lot of control.
It is also possible to "decompile" or inspect the file to get a lot of information about what it might do. This is known as "reverse engineering". This is not as complete as running it in a sandbox, but it can offer some detail before you run it.