A few decades back I attended a Met Observers Course for ATC personnel. Only certificated staff could issue Metars that were accepted by the Met Office as being 'official'.
You can become remarkably good at estimating cloud heights at varying layers when you have to do it every 30 minutes throughout your working day. Local knowledge also provides valuable help on days when the clouds are being a bit difficult. The worst is an all over grey type with no visible texture.
At night we used to use a cloud base searchlight that shone a spot of light on the clouds. The angle from horizontal was measured by an Alidade then trigonometry used to work out the cloud height.
These days it is more common at regional airports to find cloud base ceilometers zapping l@ser (now common spelling it seems)light onto the clouds and measuring the time for the reflection to return. Only drawback is that they tend only to look at one point in the sky and miss the important stratus just out of view. So they are more of an aid to the observer rather than a replacement. the use of multiple ceilometers can over come these shortcomings.