The only tool you really need is a set of max recording spring scales in the ranges (at 50% of full scale) that you need. You can get by without them by using an increasable load (the water barrel or weight plates suggested above). You need something to provide the load, but many good ideas have been recommended for that. The way you use it will certainly depend on the information you need. There are standards for testing that will make the information consistant. NFPA, UIAA, ASTM, CEN and many others have their testing protocols. In order to understand and compare strength you need to know how the rated strength is obtained.

Safe working load may vary from 25% of the breaking strength for non-critical situations to about 7% (15:1 ratio per NFPA) for supporting humans (climbers have traditionally used a 10:1 safety factor). Knots can reduce breaking strength by as much as 50% or as little as 15 or 20%.

Respectfully,

Jerry