ASP.NET MVC: What's the best way to validate the expiry/legitimate of querysting

We have a payment successful page where we read the query string ?paid=yes.

If paid=yes then we show the payment sucessfull message etc. Otherwise payment failed.

What's the best way to:

1. Validate ?paid=yes query string is valid? In other words, how can we stop people from manually manipulating query string ?

2. Set query string expiry time or set attempt (max 1)?

Thanks.