If we have a customer that deposits a check into their checking account that we suspect might be fraudulent, can we place a hold on it? if so, what type of hold would it be? Reason to doubt collectability?

"reason to doubt collectability" should only be used if you have reason to believe the check will not be paid.
did you contact the paying bank to verify funds or anything about the check? Based on what you're saying you "think" it will not be paid.
Is it a new account? have they had over drafts? is it a large dollar check?

dmk124, why do you suspect the check might be fraudulent?

As SmallBankBSA said, you need to have good reason to believe the check won't be paid. Specifically, Regulation CC says that if you are going to use the "reasonable cause to doubt collectability" special exception hold, you must have "facts that would cause a well-grounded belief in the mind of a reasonable person." In addition, Regulation CC says that "such belief shall not be based on the fact that the check is of a particular class or is deposited by a particular class of persons."

Check their overdraft history, that's the most frequent extended hold reason we place.

If you suspect the check may be fraudulent, you can always refuse to accept it for deposit and offer to send it for collection instead. Then you don't have to worry about Reg CC and if the check is fraudulent, it won't be able to be successfully collected.