You can use PayPal's API to get the balance assuming the user authenticates: https://developer.paypal.com/api/limited-release/balance-acc...

Venmo might have a similar API: https://venmo.com/docs/users/ (this shows a access_balance permission). Not familiar with it though.

PayPal's API also provide you with their transactions to search through so you might be able to actually verify/highlight your payment directly.