There is no definitive way to know about competitiveness, but as illuded to it is a combination of number of applicants and how strong those candidates are when looking against a national pool. You may get an impression to the number of candidates applying; however, you often don't know how many want a particular academy. Some districts may have lots of candidates, but West Point may be the top choice for only a few or nearly all. Next you'd be surprised how many candidates have no real shot at earning a nomination/becoming qualified. Usually a district will end up with a distribution in the quality of a candidate to include some on the below the qualification level to that one that could compete nationally. The more competitive a district, the more of those nationally competitive candidates there are.
Putting that together, you can literally have neighboring districts where there is little to no competition for a particular academy in one district and the other that ends up with a majority of candidates appointed. It's most dramatic in urban areas, but there are some states that you can see it at play. It is not a function of proximity to military bases, which although they help with concentrating the interest in a small group directly around the base (dependents of service members), most people, and high schoolers in particular, have no interaction with anyone from the base. Go 20 miles and that increases exponentially.
Add in any given year there could be variations in candidates applying and a historically easy district could become really difficult to gain admission from if there is that one stellar candidate. That is why everyone on this forum will tell you to control what you can - your own application - and make it as strong as possible while always having a backup plan.