On 26/02/2021 09:26, Ian Jackson wrote:

It won't tell you if the actual LED has failed (which is unlikely), but
it will give you pretty good confidence that the rest is OK. As Brian
has said, for the LED, some digital cameras might show you it's flashing
(even if it's not visible to the human eye).

Unfortunately they generally have filters that stop IR these days,
because without such the colour balance used to be affected.

Bill