The SNP wants to keep the pound in a formal currency union with Westminster if Scotland becomes independent.
But Mr Salmond must have a hidden fallback option because "no one but a fool would go into a negotiation if they hadn't got a plan B", Mr Darling said.
The Scottish Government must set out all of the currency options in its White Paper for independence, which Mr Salmond announced for release on November 26 at the SNP Conference in Perth on Saturday.
Mr Darling said: "The people of Scotland are entitled to know what is the position going to be if we were to vote for independence.
"You cannot assume that the rest of the UK will simply fall into line with what Alex Salmond wants. If we don't get that currency union, what is the alternative? What's Plan B?
"Now he has a Plan B, because no one but a fool would go into a negotiation if they hadn't got a Plan B. So let us hear it. We want to hear it in this White Paper because if we don't hear it then absolutely nothing he says will have any credibility whatsoever."
SNP treasury spokesman Stewart Hosie said: "Every time Alistair Darling talks about this issue he looks very silly, because it was Mr Darling himself who said in a broadcast interview earlier this year that a sterling area between an independent Scotland and the rest of the UK would be 'desirable' and 'logical', which is indeed the case.
"If Mr Darling can't keep his story straight from one interview to another, no wonder the No campaign is losing the plot."