In an N400 interview, can they ask me at the naturalization to proof my marriage was real?

At an N-400 naturalization interview, CIS may ask any and all questions so long as the questions do not exhibit racial, ethnic; religious or sexual orientation or other impermissible discrimination or prejudices and there is a reasonable connection to whether you are eligible for US citizenship. In this case, CIS legitimately wants to know if you are a person of good moral character, and where there are reasons to suspect that you engaged in marriage fraud, then, it can conclude that you lack good moral character. A denial of your application on that ground is valid as a matter of law and a court will uphold that decision as well as any other adverse action CIS takes in light of that finding including your deportation from the US. The fact that your application was filed after 5 years does not mean anything if the record shows you entered into a bogus marriage to get a green card. But based on your statements, I very much doubt that CIS will prevail in this regard unless, you either confess to CIS that your marriage was in fact a fraud or if CIS has independent evidence that you entered inot a fraudulent marriage. This is why. You and your ex-wife appeared before CIS twice (first at the I-485 initial status interview and then 2 years later when you jointly filed the I-751) That should sufficiently show that the marriage was bona fide. While it would have been nice to have kept records of your life together with your ex- wife, the fact that you no longer have them should not bar you from becoming a US citizen, especially if you are now in a new relationship. Despite these facts in your favor, CIS has a rather wide latitude in naturalization applications For that reason, you should hire a lawyer to carefully review your application; anticipate problems that may come up at your interview.and also possibly help you to recreate some documents of your former life with your ex. More importantly, I'd say that having an attorney by your side at the interview will help you a great deal

since you filed the N400 application based on 5 year LPR status, you do not bear the burden of proof that your marriage is bona fide at the N400 interview. CIS may still ask you about your marriage and if it has new evidence showing marriage fraud, it may deny your n400 on the ground that you did not gain admission legally. If your marriage is real, you need not to worry.

They can but they usually do not if you are applying for naturalization after 5 years of being a permanent resident.

Yes, they can ask whatever they want. If they deny your case, however, for an inappropriate reason, you can appeal that decision.