The short answer is, it depends on what stage you're in with the developer. If you're at an early stage before an interview it's probably not a good idea to ask the dev for the payslip. However, if you're at the final stages of someone's application after you've interviewed them then it becomes more acceptable.
Before asking for a payslip, you could give the dev some feedback on the interview before asking for payslips so that the dev knows you are genuinely interested. Be sensitive in asking for a payslip because you might be signalling to the dev that you're going to base an offer on their current salary rather than what they are worth (in the market or to your company).
On a platform level, you'd have to ask the dev directly in your conversation.
In order to obtain references, simply ask him or her directly in your messages to the dev.