Yes, Ben certainly knows his stuff where Dayak weapons are involved. Hopefully he will be able to supply us with some guidelines for making these determinations.
I don't think these weapons are very well understood in the collector world and a quick look at eBay will show you that the market is currently flooded with touristy mandaus. Many of them are quite obvious. What is a bit harder to dtermine is when we see a later example that may or may not be a practical working piece as opposed to a tourist piece. It is my understanding that heads were known to be taken by the Dayak well into the second half of the 20th century.