HOW can we easily differentiate between a preposition and an adverb in a sentence..Explain with examples..

Preposition is a word that introduces prepositional phrase. It describes relation between two things.

Adverb modifies a part of speech (verb/adjective). 

One of the main difference between the two is that prepositions take object whereas adverbs do not.

Example:

Reeta paints outside the picture.

Here, 'outside the picture' is an adverbial phrase and 'outside' is a preposition. 'Picture' is the object of preposition.