Hello,

Tollen's test: this test is used to differentiate between a ketone and an aldehyde. In this test aldehyde is oxidised  where as the ketone will not undergo oxidation. This test also known as silver mirror test. Ammonium silver nitrate is  tollen's reagent and it is mild oxidative agent. All the aldehyde give positive result.

Fehling's test: This test is used to detection of reducing sugar. It contain two solution Fehling's solution A and Fehling's solution B. Solution A is an aqueous copper sulphate where as solution B is alkaline sodium potassium tartrate. The solution B act as the chelating agent in this reaction.

Hello!!
Here is the difference between the two tests
Tollen's Test :
It is also called as Silver Mirror Test  .
In this test, Aldehydes/ketone are oxidized to carboxylic acid forming white precipitate of Ag .
Tollen's Reagent :
[Ag(NH3)2]+ NO3 , ammonium silver nitrate is mild oxidative agent which acts as  tollen's reagent.
Fehling's Test:
The test is given by aldehydes forming red precipitate of Cu2O.
Fehling's Reagents:
Fehling A solution(aq CuSO4) + Fehling B solution(sodium potassium tartrate) + NaOH.

Hope it helps.