How about a simple prism? That would tell you instantly if there's any blue light in the stream.
Edit: You might need a mask to create a slit of light so you can see the spectrum clearly.
Edit: You might need a mask to create a slit of light so you can see the spectrum clearly.
I'm sure there'll be _some_ blue light. I don't know if a prism would be high resolution enough to tell if it's a dangerous level.