I was told by another tenant that used to live in my building where I'm now renting and they told me that my apartment complex is under foreclosure, how can I find out and my lanlord isn't telling me?

It is none of your business. As long as services are being performed then there is no concern on your part. The owner may be able to forestall foreclosure or sell the property. If they sell, then you will be informed.

Why do you think you need to know or that youir landlord is obligated to tell you?

ScottGem is right, whether foreclosure happens doesn't really affect you in the short term.

But to satisfy your idle curiosity, you can find out by looking your landlord up in the court records, if it is a judicial foreclosure state. Or, in non-judicial foreclosure states, a notice of default and election to sell (or the equivalent) will have been filed in the land title records.

You can not find out, at least until it is already foreclosed on.

And agree, the renter does not have a right to know