The AMT system is parallel to the regular income tax calculation and is calculated using IRS form 6251. The calculation leads to the tentative minimum tax. Generally speaking, if the calculation for TMT is more than for regular tax, the difference is the AMT.
The calculation includes certain income and deductions, but they’re not always the same as those for calculating your regular income tax. Most notably, the exercise and hold of incentive stock options is a preference item, and is included in figuring AMT.