Now is the time to find out if you are one of the millions of Americans who won’t be able to deduct their monthly mortgage-interest payments.

For 2017, 32 million tax filers got a mortgage-interest deduction. For 2018, that number will drop to 14 million. Americans’ total savings from this break are also expected to fall sharply this year, from nearly $60 billion for 2017 to $25 billion for 2018, according to Congress’s Joint Committee on Taxation.

These...