U.S. President Donald Trump speaks during a roundtable on small business at the White House in Washington, D.C., U.S, on Friday, Dec. 6, 2019. Kevin Dietsch | Bloomberg | Getty Images

Itemized deductions

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The first two pages of a Form 1040 are a summary of taxable income a filer is required to report. The attached schedules are what can shed light on the sources of that income and the deductions a taxpayer claims. Deductions reduce taxable income based on your federal income tax bracket. Schedule A is the document taxpayers must fill out to calculate their itemized deductions, including any deductible medical expenses and state and local taxes paid.

Take note: Starting in the 2018 tax year, the deduction for state and local taxes paid was capped at $10,000 for individual filers, so there's a limit to the extent Trump — or anyone with a personal residence in a high-tax state like New York — could write off those property and income taxes. Keep a close eye on the "gifts to charity" portion of Schedule A. Donations that are more than $500 must be spelled out on Form 8283, the noncash charitable contribution form. Taxpayers must describe the donated property and provide a summary of its appraised fair market value, including art, real estate and cars.

Rental income

A sign advertises an apartment for rent along a row of brownstone townhouses in Brooklyn, New York. Drew Angerer | Getty Images

Whether your real estate empire is racking up losses or you're getting income through a web of pass-through entities, Schedule E will have the details on residential, vacation and commercial property. Trump himself uses many limited liability companies to manage different aspects of his businesses. Line 3 spells out rents received. "You can get an idea of business income, as you'd see that coming in through companies and pass-through entities, partnerships and LLCs," said Jeffrey Levine, CPA and CEO of BluePrint Wealth Alliance.

Keep a close eye on depreciation, which you can find on line 18. Depreciation is a tax deduction you can take each year to recover the cost of your real estate as you use it. While Schedule E might share the name of a pass-through entity that's providing income to the taxpayer, it may be difficult to learn the details of who ultimately owns it, said Christy Bastian, CPA and president of FVL Consultants. "You can sometimes follow through and trace entities," Bastian said. "You're looking for clues, but it doesn't mean that every return will have it." Similarly, members of a partnership aren't always easy to identify, Blank said.

Additional businesses

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Taxpayers who are running a side hustle or operating as a sole proprietor from home are responsible for reporting the income. Form 1040 no longer breaks out gains or losses related to businesses, so you'll have to find Schedule C to collect the data. This form spells out the income or loss from that small business. It also details the expenses related to running that enterprise. You can get an idea of whether an entrepreneur claimed the new qualified business income deduction — a potential 20% tax break that went into effect in 2018.

Line 10 of the 2019 Form 1040 or line 9 of last year's income tax return will spell out how much a taxpayer claimed for this. Forms 8995 and 8995-A will have additional details. This new break allows owners of "pass-through" entities, including S-corporations, partnerships and sole proprietorships, to deduct up to 20% of their qualified business income. Business owners in any industry may take the 20% deduction if they have taxable income that's under $160,700 if single or $321,400 if married and filing jointly in 2019. The IRS applies limitations over those thresholds. For starters, taxpayers in a "specified service trade or business," including doctors, lawyers and accountants, can't take the deduction at all if their taxable income exceeds $210,700 if single or $421,400 if married.

The rules are a little different for business owners who aren't in a "specified service trade or business." In that case, you get a reduced deduction if your taxable income exceeds the $160,700/$321,400 threshold but is still under the $210,700/$421,400 threshold. If your business isn't in a specified service trade or business, and your taxable income exceeds the $210,700/$421,400 threshold, then your deduction is generally capped as a percentage of W-2 wages paid to your employees. Bottom line: Nonspecified service trades or businesses Trump owns may be eligible for this deduction, depending on the amount of wages paid and depreciable property held by those businesses, said Levine.

Dividend and interest income

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