How to determine if JEST is running the code or not?

I usually have NODE_ENV=development set globally on my shell. This works for me:

typeof jest !== 'undefined'

(note that global.jest and 'jest' in global don't work, as this doesn't seem to be a global variable, just a value made available on all modules much like node's require or __filename)

jest sets an environment variable called JEST_WORKER_ID so you check if this is set:

function areWeTestingWithJest() {
    return process.env.JEST_WORKER_ID !== undefined;
}

I also see that if NODE_ENV is not set the jest CLI sets it to the value 'test'. This might be another way to check.