"To observe the difference" means to carefully notice or see the contrast or change between two or more things. It implies paying attention to specific details or characteristics that make them unique or distinct from each other.
Sentences with «to observe the difference»
Each year, regulators would look back two years to observe the difference between allowances that have been issued and those that have been surrendered to cover emissions. (rff.org)
Later, researchers observed the differences of device characteristics. (sciencedaily.com)
They are also not afraid to comment on observed differences. (edutopia.org)