As far as presentation goes, you want to be as clear and concise as possible because you only have so many characters. You can elaborate on your most meaningful activities in AMCAS (AACOMAS doesn't have a "most meaningful section"). For each activity, you need to have a reference with phone number/ email. Keep track of your hours. For paid experiences, I looked at all my previous paystubs to calculate total number of hours. Be honest and ethical. If anything you want to underestimate your hours if you're unsure about the total number (this is more so the case with volunteering work, athletics, etc).
Most proofs will be done by contacting your reference on the application (e.g. for volunteer they will call your volunteer supervisor).
Personally I would keep everything that documents my involvement. This includes volunteer sign-in/out sheet (usually signed by supervisor), payroll, certificate of completion (for research conference).
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