how do you detect this?

[Derpy]

hey

I see popular servers like FFS-GAMING are able to instantly, or within minutes give people donator status as soon as they've donated any money

How would you detect in mta if somebody sent you money like this?

[myonlake]

You have your own transaction handler that verifies the PayPal transaction and then sends a note to your server with PHP SDK or simply just their own curl call. With these you are able to make an almost waterproof system, though keep in mind if they claim refund on their donation you need to check that too...