I did not claim my 401k cashout (which was my ex husbands and I got it in the divorce). I honestly thought the 20% taken out was all I had to pay but doing research I see I was wrong. I know I HAVE to pay the taxes and I will. My question is - without the 401K added to income I am due a refund, will the IRS catch this now before they send me the refund or later and send me a bill?
Thanks

Probably later. But if they catch it you will probably have to pay a penalty and interest on the difference.

I suspect you did your taxes yourself. If so, I would strongly urge you to take this to a tax professional. Since the distribution was possibly the result of a QDRO (you said you got in in the divorce), the tax implications are different.

You are sent notifications of income prior to 1/31 in the form of W2s and/or 1099s. Generally all of these notifications need to be reported on your return.

As an aside. I removed the thread I moved. I did not realize you had posted your own question. Generally we do not allow reviving old threads. But, in reading that thread the OP had stated that he HAD gotten caught.