and how do you know it is the genuine stuff?

Depends what you mean by genuine. I have used Bike tart in the past and it seems to hold up well, hasn’t discoloured after 3 years. When they were out of stock recently I tried an e-bay seller paragon tapes, what he described as aircraft grade 0.4mm tape and that is also fine

Edit: I should also say that the Bike Tart tape hasn’t shown any signs of peel or shifting after 3 years on my Orange 5

Get the 3M stuff.

http://www.tweekscycles.com/Product.do?method=view&n=3553&p=16495&utm_source=Google&utm_medium=Base&utm_campaign=Frame%20Protection

I used paragon tapes on ebay- cheap and good quality.