How can I determine if I was inappropriately awarded an AAM in lieu of a MSM?

The AAM can be awarded for both meritorious achievement or service, so while an MSM recommendation downgraded to an AAM is in my experience unusual, it's not by definition improper. So the short answer is (without more detail like the 638 and comments) No. See the excerpt below from AR 600-8-22.
3–18. Army Achievement Medal
a. The Army Achievement Medal (AAM) was established by the Secretary of the Army, 10 April 1981, as announced in Department of the Army General Order 15, 1990.
b. The AAM is awarded to any member of the Armed Forces of the United States, or to any member of the Armed Forces of a friendly foreign nation, who distinguished himself or herself by meritorious service or achievement of a lesser degree than required for award of the Army Commendation Medal.
c. The AAM will not be awarded to general officers.

SSG Jaime N. Garcia Sr very easy to figure out. Do you have the 638? See who down graded it and at what level. When they downgrade, there is a box to write what to down grade to. Just because it says meritorious service in a narrative doesn't mean it was meant to be an MSM. As SFC Brian Johnston States many units have a lower level delegated disapproval authority, if the subordinate commander closest to the recommended soldier didnt think they should receive that award, it will be DOA at the approval HQ. It should be delegated in writing and it very well might be, but it's not like everyone in the command gets a copy. As long as the S1 has it and the subordinate commanders know what the procedure is, it ends there. This was the policy in the 101st circa 2007-2009. Started with Eagle 6 for Air Medals, MSM, and BSMs. The Brigade Commanders followed suite for ARCOMs. I was a BN XO, never saw the Memo, but it was so.

Who signed your award certificate? With a delegated downgrade authority, it should be the person who downgraded it. Without, it would have to go all the way to the approval authority. Example: I was put in for an MSM in Korea. My O6 commander recommended approval. Hand walked it in to the two star. Was told no face to face. The G1 prepared the ARCOM certificate and the two star signed it. I have the 638 that shows the approval authority downgraded it. Irony is more energy was expended to disapprove and downgrade the award than if he just signed the MSM as is. Water is wet, sky is blue. It is what it is.

Unfortunately 90% or more of awards in the Army are subjective. (Things like the Purple Heart are fairly objective, even though occasionally subjectivity rears its ugly head.) There is no such thing as the "appropriate" award for any given action or series of actions. What warrants a MSM changes from Commander to Commander. Sometimes even from day to day.

That being said, SFC Brian Johnston hit the nail on the head regarding the downgrade (I'd tag ya, battle, but I don't know how).
I will caution you, though.... the approving authority has three options: approve, downgrade, or disapprove entirely. Fighting it may mean you end up with nothing. (Of course, if CO, BN, and BDE support, it can always be re-submitted as an ARCOM.)
Finally, I will say that, over time, things usually even out. I watched three of my Soldiers get AAMs as EOT awards when they DEFINITELY deserved ARCOMs (IMHO). I was absolutely p*$$ed, and went ranting and raving to 1SG. He told me to calm down and hold my horses. Three months later, they all got ARCOMs for PCS, even though their non-deployment activities for the unit warranted an AAM, at best (IMHO).