Safelights are designed to emit a narrow red spectrum that black and white paper (and orthochromatic film) is insensitive to - you can see the materials without destroying them.
Most older safelights use a dim incandescent bulb with a rubylith filter to block out all blue and green light. Because incandescent bulbs produce black-body radiation, there is non-zero blue and green light despite the deep orange appearance. If the filter is old and faded or there are light leaks in the safelight housing, the light will cause fogging problems with your prints, though perhaps only very subtly as the loss of highlight contrast.
Some newer safelights use LEDs, the betters ones of which emit pure monochromatic light. Their perfectly monochromatic nature means that no filtering is required to make the light safe, therefore the light will remain perfectly safe.
Safelights should be placed so that they point towards the (white) ceiling, which will diffuse the light throughout the room. Try not to place them directly above the enlarger or developing trays and in no case should a safelight come within 1m of paper.
To test that you have no fogging from safelights:
You should see no outlines of coins on the sheets. If you can see pale circles then either your darkroom has a light leak or the safelights are not safe. The more coin outlines you can see, the worse the light-leak and the faster your paper will fog.
The darkroom should be dark enough to give you at least 30 minutes of fog-free operation and 60 minutes is quite achievable.
Red safelights should not be used when printing RA4 because that paper is sensitive to red light; a red safelight will cause strong cyan fogging.
Dark green safelights are available for RA4 use but they are difficult to see by and can be used for only a very short period of time before causing fogging. They're designed to emit light in a narrow spectrum where there is a notch in the paper's sensitivity.
It is possible to purchase "night vision" goggles with a digital video camera on the front, some IR LEDs for illumination and a small screen inside the goggles for you to see the camera output. While expensive ($200-500), these goggles allow you to handle panchromatic and colour films and see what you're doing without fogging them. They're particularly nice for loading film holders and spirals, and to develop by inspection.
If the LEDs make a visible red glow, you can buy 900nm LEDs (well outside the visible band) and with a little soldering, replace the LEDs on your goggles for greater safety with red-sensitive films.
Ensure that light from the screen in the goggles cannot escape, as it will certainly fog films and papers.
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