The other way is to open cmd and type "ping google.com" and then post the result here and we'll help further. I highly doubt BT will know if you're on fastpath on FTTC being a different DSLAM to ADSL.
Given that interleaving adds latency (see What is interleaving? What are its pros and cons? Can I ask for it to be switched off?), if you monitor latency to an Internet based host using something like ping, you could make a somewhat educated guess as to when it's on. No guarantees though.
This is one of the reasons you see people using third party routers.
Looks like you've crossed post. If the user can also tell us his general location, that will also help us.
can you determine if you are fastpath or interleaving from HH5 stats?
@jdsmithNo you can't tell exactly but if you ping let's say bbc.co.uk, and get ~6ms that's either fastpath or G.INP. This isn't always reliable depending how far you are from the server.