How to check the straight razor for sharpness?

I’d like to try shaving with a straight razor. I found one, and it seemed pretty sharp (cut the hair on my arm). However, when time came to shaving my face, it pretty much scraped off the shaving cream without really touching the beard. On a positive note, I didn’t cut myself! Unfortunately, I didn’t shave either. I sent the blade off to get professionally sharpened. But now I am curious, is there any way to tell how sharp the blade really is before trying to shave?