foreign spouse income

Whether or not foreign income is taxable in the US is usually a function of the tax treaty (if one) between the US and the country from which the pension or other income originates. As the IRS says:

"As a general rule, the pension/annuity articles of most tax treaties allow the country of residence (as determined by the residency article) to tax the pension or annuity under its domestic laws. This is true unless a treaty provision specifically amends that treatment. Some treaties, for example, provide that the country of residence may not tax amounts that would not have been taxable by the other country if you were a resident of that country. In some cases, government pensions/annuities or social security payments may be taxable by the government making the payments. " (see The Taxation of Foreign Pension and Annuity Distributions).

While many tax treaties are similar, they can all be unique. You can try to do your own research, starting at Tax Treaties. However, they are convoluted and not easy to understand even for tax professionals. Therefore, I would encourage you to consult with a tax professional who has a lot of experience doing US returns that include income from foreign sources.

As for proving the numbers, the IRS will likely accept the documentation that you have: either reports from that country that you have received or deposits into your checking account.