Formal:
* Verify: To check or prove the truth or accuracy of something.
* Authenticate: To prove the genuineness or validity of something.
* Substantiate: To provide evidence to support a claim.
* Validate: To make something officially acceptable or approved.
* Ratify: To formally approve or agree to something.
Informal:
* Check: To make sure something is true or correct.
* Double-check: To check something twice to make sure it is correct.
* Verify: To make sure something is accurate.
* Ensure: To make certain that something will happen.
* Guarantee: To promise that something will happen or be true.
Other:
* Assert: To state something firmly.
* Declare: To announce something formally.
* Affirm: To state something positively.
* Prove: To show that something is true.
The best synonym to use will depend on the specific context of your sentence.