It's pretty easy for me. I'm very sensitive to it and an increase always presents instantly with symptoms like heartburn, reflux with extreme "sour" fluid, pain/discomfort etc.
A decrease, experienced when taking omeprazole or stuff like that in substantial amounts or any other kind of meds, food etc that absorb, reduce or do whatever to combat the acid makes these symptoms dissappear but also leads to noticeable changes in digestion. It seems like the stomach has a hard time digesting anything when there's not enough acid, which seems logical. Then, some kind of reflux can also appear but it feels completely "un-acidic".
Uff, having a hard time explaining this in english right now, i'm a bit confused at the moment. Hope i somehow made it understandable.
edit: Forgot the important point: After using ethylphenidate for a while i noticed that i din't take any anti-acids or stuff like that for a while which is strange for me, esp. since i live an unhealthy lifestyle at the moment which usually always requires these meds because bad food (or not enough food), lack of sleep, smoking too much etc have always caused stomach issues involving acid for me. And that i completely lack any symptoms like that just made me wonder what the reason for this might be. But i know that i make some pretty far fetched guesses here.