How to understand spoken French: il pense/parle vs. ils pensent/parlent
How do you hear the difference what is meant?

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The context is important, if you understand the general meaning of what is said, you will know if the subject of a sentence is singular (il parle/pense) or plural (ils parlent/pensent).

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You can't hear a difference, they are homophones. 
As "Languages With Mona" said,  you have to be aware of the context.