Choose a calm moment and begin with an area your cat already accepts being touched. Keep the session short enough that you can stop while the cat is still comfortable. Work gradually rather than trying to inspect the entire coat in one sitting. Use your fingers gently to notice changes in texture, then use an appropriate comb only where it moves without pulling. Watch for tension, tail movement, turning towards the hand or attempts to leave. Allow the cat to move away instead of holding it until you finish.
If you repeatedly find the same difficult area, ask for a professional demonstration or assessment. A written note of the location can be more useful than continuing to test a painful knot. Building tolerance over many calm sessions is preferable to completing a thorough inspection through force. Separate the goals of checking and removing a knot: finding a tangle is useful progress even when a professional needs to handle its removal.
For product comparisons alongside this advice, see Cashmere Comb grooming set and Reusable pet-hair roller. The details below explain their sizes and uses.
Further reading: cats.org.uk; pdsa.org.uk; rspca.org.uk; bluecross.org.uk; cats.org.uk.
Cashmere Comb grooming set
Reusable pet-hair roller