Short answer
A person should disclose material information that could reasonably affect the other person's decision to marry.
Islamic Guidance
What Islam teaches
Deception and concealment of significant matters are inconsistent with honest dealings.
Practical Guidance
What you can consider
Be honest about marital history, major financial obligations, significant responsibilities and other material matters.
Sources
Qur'an 33:70; Qur'an 5:1; Qur'an 4:19.