The scammers are pretty brazen and try to convince others that their scams are true.
Here are some examples:
1. A scammer vouches for a scam that he promotes, for example: for the Email fraud: qualifying for the Cadbury Foundation cash aid, he adds a comment saying:
Name: James Arnold
IP Address: 220.127.116.11
Subject: They are true.
My Friend made his money from this same email, last year.
They are for real. don't say what you do not know.
Of course, it is fake no matter what the liar says.
2. Claim to be law enforcement (police, FBI) and vouch for the fraud they are promoting. Here are some example emails: Internet scams claiming be law enforcement (police, FBI) and verifying other frauds.
3. Claiming that a scam in the name of the Sultan of Brunei is true. For example: here. Until he was exposed by his IP address being in South Africa.