My husband and I are separated and he filed my taxes without me knowing and said we had to pay. How do I verify that

I suggest that you talk to an attorney.   A joint return must be agreed to and signed by both spouses or it is a fraudulent tax return.

you'd have every right to report your spouse's actions to the IRS. You can do this by calling the IRS 800-829-1040 option 2 and directly reporting the fraudulent return. However, there's no guarantee that the agency will take action on your complaint. ask the agent  if you can file your own separate tax return. The IRS would then have to review both returns and hopefully it will be obvious that your spouse committed an improper act.

do not pay or send him any money.