A call to the issuing bank will verrify if the check is a good one.
They put serial numbers on them things, and I hear tell banks have tem newfangled computer dojiggies.
As far as the fake 800 number goes, banks have ABA numbers and there are lists and books put out by the ABA as to who to call which your bank will have and be aware of.
http://www.aba.com/Consumer+Connection/cashierscheckfraud.htm
from the FDIC:
Fake Cashier's Checks
A cashier's check—one where funds are already set aside in a special account at a bank—can be a safe way to receive payment for that used car you're selling or items you auction over the Internet. Unfortunately, FDIC fraud investigators are reporting that fake cashier's checks are getting easier for crooks to produce on personal computers. In one example recently reported to the FDIC, an individual lost a classic car worth $41,000 to a thief who used a counterfeit cashier's check. How can you protect yourself?
"First, insist on a cashier's check drawn on a local bank or a bank that has a local branch," says FDIC fraud investigator Gene Seitz. "That way you can take the check to that bank to ensure it's valid." If you can't get a cashier's check from a bank with a local office, Seitz says, confirm that the out-of-town cashier's check is good by calling the bank.
Use the phone number from a reliable source, such as directory assistance. "Don't depend on the phone number given to you by the buyer," he warns, "because if this is a fraud, that phone number could just put you in touch with a partner in crime." If you're not sure where the bank is located or if it is legitimate, check the FDIC's "Institution Directory" on the Internet at
http://www2.fdic.gov/idasp/