Even though it has been known to bring on laminitis in some instances, some vets will use "The Dex Test" to test a horse for cushings or other metabolic issues.
If you didn't run across this link when you Googled, it may be helpful. https://www.fairfieldequine.com/site...es/dex_rdm.pdf
The bottom line is that Dex is not something that should be used on a horse in an off-hand/ho-hum manner.
If you think the Seller would be co-operative, I would just flat out call and ask why the horse was given Dex - hopefully they would know that Dex was administered to their horse but don't be surprised if they honestly don't or feign surprise