When a payment is being sent to you via Plooto you will receive an email letting you know it is in process.
If you do not recall receiving an email, or may have missed an incoming email, find any payment notification that is coming from the same payer. On the notification emails is a link that says "View my payments".
If you click on the View My Payments link on any of the emails you have received you will see your future and past payments.