Hold the plant to the sun, then…
During late autumn through winter, sun light comes into rooms with lower angle compared to warmer seasons.
Hold the leaves or sprigs or maybe even entire plant if it is possible, to the sun.
Then observe shadows appear on the leaves carefully. Scale insects, aphids and spider mites on the back of the leaves appear as shadows. Remove them with brush or pinch them with your fingers, or whatever necessary to get rid of them, before they breed and increase their numbers by multiplying.