It will have the product code stamped on somewhere but you don't have to buy the exact same one anyway, just make sure you get Shimano* or Campagnolo, (whichever you run), and FWIW I'd stick to the same ratio cassette, (eg: 12-25), just to avoid any issues with chain length.
If in doubt just count the teeth on the largest and the smallest sprockets and get the same. Also, as you've gone down the extra wheel route anyway I'd stick the new cassette on the road wheel and the old cassette on the turbo wheel.
*Sram cassettes are fully compatible with Shimano as well.