how do we distinguish between formic acid and acetic acid...????

Your friends tried well.
The most common test to distinguish between formic acid acid and acetic acid is silver mirror test.Formic acid will produce silver mirror on reacting with ammonical silver nitrate. Reaction is as follows:
HCOOH + 2[Ag (NH3)2] NO3 → 2Ag + CO2 + 2NH4NO3 +2NH3
 

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Acetic acid will give yellow precipitates in iodoform test while formic acid would not.

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for lab exams try smelling it.

if it has a vinegar smell it is acetic acid.

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Formic acid gives positive tollens test, fehling's test and benedict's test. Acetic acid doesnt give any of the above.

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