Start with the pay stubs. These show the numbers for taxable income and the taxes withheld for each paycheck. They also specify the Year-to-date numbers for income and withholding.
If there are things that aren't on a regular paycheck for example a moving expense, or a bonus those may be handled with a separate check, but the associated stub should show if it was taxable, or if any taxes were withheld.
Sometimes these extra items are added to a regular paycheck, but the information on the stub should specify if it is taxable income and if any taxes were withheld.
Some companies pay for excess PTO. They don't have use it or lose it, they pay you if you have PTO over a specific limit. That money is taxable, but again there would be a stub.
Another item that can account for a small delta, is that if the company paid for a large life insurance policy. If the amount of the coverage exceeds $50K, then the cost of the excess coverage is taxable income. For most situations that isn't a large amount of "extra" income, but it can be if for example the company pays for a $1 million policy for every employee.