If you overdraft a prepaid card and have overdraft protection on the prepaid card then the transaction would go through and you would be charged an overdraft fee which is often $30.00 although the fee can vary.
However most prepaid cards don't have overdraft protection and if they do you often have to opt into the overdraft protection.
Most prepaid cards don't allow you to spend more than you have and will simply be declined and not allow an overdraft.
If you try to spend more money than the amount you have on your prepaid card then the transaction will often simply be declined.
If you do have overdraft protection then the bank that issued the prepaid card will cover up to a certain amount and then charge overdraft fees.
Most prepaid debit cards do not allow you to overdraft your account, and those prepaid debit cards that do permit overdrafts typically have additional requirements or restrictions like direct-deposit enrollment or preventing your balance from falling below negative $10.00
To ensure you have enough on the prepaid card before spending lookup the account online so that you won't have to worry about it being declined.
If you have money that you can load on the prepaid card then you should do that before using the card so that it will go through.