Simple test. Install the stem into a test bike (a junker). Slide some old handlebars in and tighten. Lay a piece of plywood over stem and bars. Put a scale under the front fork; load up 200 lbs, and now add extra weight with hands on the bars. Push hard up and down. See if you can max out the scale. If that works and the stem shows no signs of deformation, you're probably good to go. Normal working load is about 40% of body weight on the front wheel. The handlebars get maybe 25%. So if you're max'ing out the scale (and mine goes to 400lbs) and bouncing a little up and down hopefully flexing the bars, you're probably going to be okay with a pretty big safety factor.