Flex is sometimes marked (mains will typically be marked 300/500, or H05VVF or similar for mains flex in the UK).
However, you can check the manufacturer's spec, and for the sources I can find online, the manufacturer rates trailer cable at 50V.
It might work fine at mains voltage, but you have to prove that, as the manufacturer doesn't (and the manufacturer probably won't provide you with enough details of the manufacturing process for you to prove it yourself).
Also, ISTR that the earth wire is never allowed to be covered with anything other than green/yellow for mains wiring, and given there is no such conductor (nor a bare conductor which would also be OK), even if you can prove the cable is OK for mains use, it could only be for appliances which were double-insulated and thus not earthed.