Polarity of my special

[sales@johnsweet.co.uk]

Many years ago i purchased a Cambridge special which was a non runner. the intention was to have it running in a short period of time. This never happened and many years have rolled by. I am now however determined to get it on the road. ( whilst I am still able to get in and out of it) I have however come across two problems which have got me stumped,they are as follows. As I bought it as a non runner and without a battery I do not know if it is positive or negative earth or indeed has a previous owner changed it from 6 volt to 12 volt. The "big red book" really does help that much as the year of manufacture is 1936 and there were lots of changes about that time also a lot of people made the switch from 6 to 12 volt. 
Is there a sure fire way to answer these questions with the limited information I have been able to offer. Thank you in advance