Anhydrous Amiloride HCl is commercially available and is stable in air. This suggests that you should be able to remove the waters of hydration, however many that may be, by repeatedly heating in a drying oven until a constant weight is reached. If you know the weight before drying you could of course calculate the original degree of hydration after the final drying, if that is of interest.
If you need to further verify dryness, you could have the dried product analyzed by Karl Fischer water anaysis (~$50). Just be sure to seal the sample under a dry atmosphere and label "HYGROSCOPIC" all over the sample and submittal paperwork!
Once you have the constant-weight product, stored under a good desiccator, just use the formula weight of the hydrochloride salt, 266.1, to calculate the concentration of your Amiloride HCl solutions.