Ehm, I'm not so sure about safety of simple sends... Please check this example:

Suppose Alex wants to sell a physical gold bar for mastercoins. One day, buyer comes to his place, wants to buy gold. Sends Mastercoin simple send transaction to Alex's address. They wait for three confirmations. Transaction shows up in all Mastercoin explorers, shows Alex's balance correctly. Alex is super-suspicious, so he checks transaction chain, and finds that mastercoins he received are connected to exodus transaction through 3 simple sends. Alex knows that he can trust simple sends to work, so he believes it is OK and gives gold bar to buyer.

A month later, Mastercoin protocol is upgraded and Alex's balance is 0. HOW???

Well, it turns out that buyer was clever: he marked his transaction as savings (or sent any other currently-disabled Mastercoin transaction) before sending a simple send.

So at the time transaction was made, none of mastercoin explorers knew how to interpret mark-as-savings transactions, so they ignored it and assumed that simple send is correct. After protocol upgrade, mastercoin explorers now understood the old mark-as-savings transactions, and now it appears that simple send is invalid.

From what I see, it isn't enough to check whether mastercoins came to your account via a simple send, you have to:

1. inspect transaction history of each address mastercoins have traveled through before they got to you
2. identify all mastercoin transactions
3. if history has anything but simple spends (particularly, if it has transactions you cannot understand), you should reject the offer

Is there a tool which does this?

Or is there something in the spec which prevents re-interpretation of old transactions (i.e. so that ignored transactions won't ever be considered as meaningful)?