A bulk of the "presumptive" issues are Agent Orange related - various cancers, diabetes, ischemic heart disease etc. This means the only thing you need to verify is that you were boots on the ground in Nam and that you presently have the condition you're filing for. The current Camp Legeune water fiasco is another recent "presumptive" situation. If not presumptive, then the onus is on you to provide proof that the issue you are claiming was caused by military service. That said, a lot of times the issue may be one that you would have acquired even if you weren't in the military but because you were in when diagnosed, you're good to go. I filed a number of claims for vets who were claiming what I thought were issues brought on by their own stupidity (getting shit-faced and doing a face plant on a curb, etc.) but based on VA rules, they were eligible for a rating. Also remember, a claim can be reopened if the condition worsens - Type II diabetes requiring medication to control rather than just diet modification - and the rating increases.
My advise would be to contact your County Veterans Service Officer when you ETS, or retire. They'll sit down and go through your records and advise you accordingly. (Be advised, however, some are better than others)
The military has tried to get people to file claims before they ETS or retire, claiming that it would speed up the process. From my experience, don't do it. Reason being is that those claims go to a VARO that is lousy at their job and claims set for over a year. Milwaukee VARO is actually pretty darn good compared to some other black holes. Wait till you get out to file so your claim will be processed in Milwaukee.
I don't know of many people from Combat Arms or who worked on a flight line who don't have tinnitus or some degree of hearing loss. Qualifying on a range is exposure to noise. Hearing aids are expensive.