Caterpillars can be seen at times but mostly it is the damage the cause that gives indication they are around. Most leaves with irregular cuts or leaves totally removed have most likely been eaten by Caterpillars. There are several types which prefer roses and all are quite small.
As their life cycle is quite short unless a huge amount of damage is being caused it may be to just live with it. If you must control several specific bacterial caterpillar sprays exist which will be safe to other insects. ‘Success’ or ‘Dipel’ can be used to control as well as ‘Eco-neem’ as a more natural method.
Caterpillars can also ‘drill’ holes through the developing buds which appear as petals with holes.