My employer reimburses me for up to $50/month for a gym membership. When I put in an expense report, they give me a $50 check. There is no tax withheld. Do I need to report this as taxable income?
Yes, it should be included as taxable wages on your W2. While a gym at work (i.e.: on premises) is a non-taxable fringe benefit, when you're being provided an actual gym membership elsewhere - it is considered wages.
If this is a big company, they should have an HR department that you could ask and that would have people knowledgeable enough to give a reliable answer.
But I believe the answer is "yes, it's taxable". I had a job a few years ago where the company paid for YMCA memberships, and that was taxable. They showed it as income on my pay stub and withheld taxes for it.
As @blm notes in his comment, anything of value that your employer gives you is taxable, except for things that are specifically declared tax exempt. The list of tax exempt things is pretty long and includes lots of familiar benefits, like employer contributions to health insurance, etc. But gym memberships were not on the list last I checked.