Light spacing is published in the AIP entry for the appropriate aerodrome.
For aerodromes with human observer RVR, the whole system is calibrated on a regular basis and only observations made from the "calibrated" observation point are allowed.
If there are no RVR reports available and you are making an assessment from the cockpit then a call to ATC will give you the spacing at any aerodrome - even if they have to ring the aerodrome manager for the info!.
Regards,
DFC