When you take the drive out of the PC, you can see this in the back
(pic only for illustration - not the same on all models!)
The other thing you'll be able to spot is the model number and make of your drive. If you search for it on the manufacturer's web site, you'll see what the jumper settings should be. If you're lucky, they printed what the settings should be on the sticker on top of the drive! So set that to master and you should be good to go.
I wish you'd asked prior to buying the drive whether the enclosure box was for ATA or SATA. Master and slave is only relevant for ATA (parallel ATA aka PATA aka IDE aka EIDE) (as opposed to SATA aka serial ATA), so you know the enclosure is ATA. ATA cables allow 2 drives per cable (one master, one slave). SATA cables only have 1 drive per cable. But hopefully you bought the correct enclosure for the interface of your drive.