To begin with, all rods and reels have line ratings that indicate what size line should be used with that piece of equipment. A general rule is to make sure your chosen rod and reel fall within each others line rating parameters. If your chosen rod has a line rating of 8lb - 14lb test line, then choose a reel that is rate for 8, 10 or 12lb test line. There are exceptions to this rule but it is a rather reliable in pointing an angler in the right direction to making a rod and reel choice.
If experimenting with reels isn't possible, the next best choice is to accurately weight something that you can affix or hang from your rod, simulating the weight of a fishing reel. Weigh various small, household objects on a reliable scale that weighs in ounces. Note the weight of each object and then hang them from the reel handle using a string, nylon stocking or something similar. Once you find an object that balances properly, note the exact weight and you have a perfect reference for your reel purchase.
