If you do not have access to a second meter to compare against, here is the only thing I can think of to try to compare...
a. Get an 18% gray card, meter it with Spot mode in your camera meter, shoot a photo of that card. That becomes your 'proper exposure' density to compare against the next step.
b. Meter the flash with your Kenko, take a flash photo of the 18% gray card at that indicated combination of aperture and shutter speed.
c. Compare density of shot A with density of shot B...they should be fairly similar.
BTW, I have found that my own 40D spot meters the 18% gray card at the same light reading as obtained with my Minolta Autometer Vf in incident mode (that same meter later began to be marketed by Kenko)