Hi Ian
I have tried the white vinegar test and it appears to work but to be only part of the answer. If there is a bit of pointing that has fallen of or is so loose that it falls of this could be cement pointing with a trace of lime mortar on the back – it would therefore fizz anyway so you also need to look at the material and try to assess. I don’t find it all that easy. This YouTube is possibly the one referred to.
http://www.youtube.com/watch?v=Fh0Ad3FKDrE
This from Cornish Lime suggests analysis for Listed Buildings Consent verification