I'd use weights from a weight set. Back when I had a home weight set, I could stack 350 pounds on there. The weights would tell me how much tension is on the rope. I'd also try to simulate a knot-less rope, as much as that's possible. Round turns around a large diameter pipe would simulate the no-knot. My old weight set had a weightlifting cage. So, I'd rig the pipe to the cage somehow.

Anything higher than 350 pounds, then I'd take the manufacturer's word for it. At that weight, you're messing with a dangerous amount of tension (not worth risking serious injury). Then, I'd multiply the manufacturer's posted limit by 2/3, and use that as my working max. For example, If the manufacturer says the break strength is 900 pounds without a knot, then I wouldn't push a Bowline past 600 pounds (900 x 2/3).

On the other hand, if I put some fishing-type knot in the rope (which would be a huge knot in regular rope), then I might use that rope at a higher tension than 600 pounds if I absolutely needed to do so.