That's simple. i ask for the initialed section on the last page of the  AZ contract that says - seller accepted another offer. Done.

And....I have not had this experience for many years - that an offer of one of our buyers wasn't presented, probably since the short sale days back in 09 to 11.

We have a form for that.  And the one time I asked for it to be completed, I'm pretty darn sure that the agent signed it for him AND the sellers. 

Our Board added a receipt box to all offers to eliminate this question. I would ask for a signed agency disclosure or rejection written and signed on the offer if you are questioning if it was presented. Many buyers want to know their offer was reviewed. 

Please know that if you reply to any of these messages, your "anonymous" cover will be blown.  Replies are not anonymous.

From the REALTOR Code of Ethics, Section 1, Standard of Practice 1-7, in part,

Upon the written request of a cooperating broker who submits an offer to the listing broker, the listing broker shall provide, as soon as practical, a written affirmation to the cooperating broker stating that the offer has been submitted to the seller/landlord, or a written notification that the seller/landlord has waived the obligation to have the offer presented.

Tell the agent the buyer is requesting proof the seller reviewed their offer. We don't have a place for that on our contracts so they may need to initial somewhere stating they reviewed the offer.

Ask the listing agent to have the sellers sign the rejected status on the bottom of the offer.

In CA we have a section the seller can initial.  That's about the best we can do.  

You can ask but the seller has no duty to comply! 

UGH. You can ask for a receipt, you may not get one. I try to be the adult in the room.
"Received, and in the hands of the seller."

I can tell you offhand the names of a couple of agents in my local area who got suspensions for failing to present an offer. Strangely these things can come out in the wash.

A disclosure seller reviewed the offer.

We never had this experience.  It looks like others members answered your question.  Good luck.  A

What kind of proof are you looking for? I've heard some agents request to present their offer to the seller with the seller agent present

You answered your own question. The key word? "nicely"! Get creative because this will meet with resistance. Something like I either need it for the files, my office manager or my clients feedback folder. State you can pick it up and thank you so much. Perhaps we will meet again out there?

Everyone has provided a good suggestion... nothing I can add to this. 

I have never suspected an offer was not presented, ever.  And, if we're in a Multiple Offer scenario, all offers are recorded due to a history of "Phantom Offers" being played out.  We stay on top of all nonsense up here  🇨🇦

You can ask about the status of the offer. If the agent is a true professional, an answer can be gained, if not, moving on quickly will help achieve the client's goals instead of getting stuck on this!

I just ask for the page of the contract signed by the seller rejecting the offer. 100% of the time my email request is ignored. That does not stop me from asking. 

Our forms committee eliminated the box for rejection or counter, and so now we do need to ask.

Our contract has a section for that. You can request the LA have it filled out by the seller. This usually ends up as a slight to the agent no matter how you handle it. Can't worry about it - you work for the buyer. 

Reach out to the other broker for a start.

I don't see anything wrong with asking for a confirmation, in writing. It is one of the "holiest" obligations of a listing agent to present all offers.

You go to the website where ALL offers are seen by the seller and all the offers are seen by other buyers. Every one is anonymized, like you are right now.
Would that not SOLVE the problem?
And right there in the lower left, the seller checked the box labeled 'REJECTED.'
Now the buyer wants to know "WHY" it was rejected.
Even in an open ledger system the loser is never assuaged, always a victim.
It is not uncommon for a listing agent to have the seller initial every offer. You however are not ENTITLED to that document.

Sadly, "presenting it", and "presenting it properly" are two different things. I am pretty sure I just experienced the latter... BUT... my clients ended up with a better (i.e. newer and lower-priced) property in the end. Sometimes these things happen for a reason.