This can be done with a standard gemological triplet loupe, however a proper microscope aids in defining the types and traits of inclusions present within a stone. It's a common saying among many jewellers and collectors that stones of SI2 clarity and below more commonly show eye-visible inclusions that you can view with unaided vision, however this is just a general observation and there are always exceptions.
The stone above shows some prominent feather inclusions to the lower left side, which may be spotted by eye if careful or keen enough. The tiny inclusions at the center though would be thoroughly missed unless you were making proper use of a loupe or microscope. Certain inclusions weigh more heavily compared to others in the hierarchy of quality grading. Pinpoints for example, pose no durability threat, however feathers- which are essentially fractures or breaks in the stone, if large enough can compromise some areas of the stone's durability.
