Get one of the security cameras with the ring of IR leds around it for viewing stuff at night, then youre certain the CCD/lenses dont have an IR blocking filter. Disable the IR led ring and then at night you can see IR emitters without throwing out any of your own.
If you put an IR pass filter on the lens you could view only IR during the day too, though you'd still be competing with the sun.
An IR pass filter on a DSLR makes for interesting photographs of translucent materials and plastics in general. Some things become clear (like fronts of VCRs) and some become opaque or tinted. Dyes and inks look a lot different, a wood table looks fine, but something with a cheap printed veneer on it may look totaly blank. Makes you wonder what it's like for species that can see IR or see UV.
