Te test proposed by @bvy is a nice one.
I would just add a couple of tips.
Photographic paper reacts to the blue component of white light. The amber light needs to have a very thick layer of filter, so use red that blocks more the green-blue components.
A visual test before you make the actual photographic test.
With only your safe light on (with your layers of cellophane paper), try to see some magazines with a clear green and blue bright colors over black. If you can not see the green and blue at all, you are probably safe.
You can also take a photo with a digital camera and watch the RGB histogram. It should show only red component. A tiny little green is acceptable.
Some images as a reference. Taken with a DLSR. (I tried using a phone but it is useless. The image is totally overexposed, no white balance... So it renders the lights white)
Red safelight. Settings: Iso 100, 1/200, f5.6 White balance>Sunlight.
Amber safelight. Settings: Iso 100, 1/200, f5.6 White balance>Sunlight.
Red cellophane paper (4 layers) over a small white led light. Settings: Iso 100, 1/200, f5.6 White balance>Sunlight.
Red cellophane paper over my lens (4 layers) pointing to an incandescent 60w bulb. (It is hot and too bright, so do not use it It is only a test to see the RGB channels) Settings: Iso 100, 1/2000, f5.6 White balance>Sunlight. I did not focus on this to blur the filament.
All have on this photos a bit of blue on the far left, but it is negible.
The histogram I am posting is a bit tricky because I did some not controlled tests. It is just a reference, but you can simply open the images on Photoshop and take a look at the channels, specially Blue and Green ones, and see that they have almost no component there.