(i)
Methylamine and dimethylamine can be distinguished by the carbylamine test.

Carbylamine test: Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form foul-smelling isocyanides or carbylamines.

Methylamine (being an aliphatic primary amine) gives a positive carbylamine test, but dimethylamine does not because it is a secondary amine, and it does not give a carbylamine test.

The reaction is as follows:

(ii)
Secondary and tertiary amines can be distinguished by allowing them to react with Hinsberg’s reagent (benzene sulphonyl chloride, C₆H₅SO₂Cl).
Secondary amines react with Hinsberg’s reagent to form a product that is insoluble in an alkali. 

Tertiary amines, however, do not react with Hinsberg’s reagent. The reaction is as follows:

(iii)
Ethylamine and aniline can be distinguished using the azo-dye test.

A dye is obtained when aromatic amines react with HNO₂ (NaNO₂ + dil.HCl) at 0-5°C, followed by a reaction with the alkaline solution of phenol. The dye is usually yellow, red, or orange in color.

Aliphatic amines give a brisk effervescence due (to the evolution of N₂ gas) under similar conditions.

The reaction is as follows:

(iv)
Aniline and benzylamine can be distinguished by their reactions with the help of nitrous acid, which is prepared in situ from a mineral acid and sodium nitrite. 

Benzylamine reacts with nitrous acid to form unstable diazonium salt, which in turn gives alcohol with the evolution of nitrogen gas. The reaction is as follows:

On the other hand, aniline reacts with HNO₂ at a low temperature to form stable diazonium salt. Then diazonium salt reacts with the alkaline solution of phenol. The dye is usually yellow, red, or orange in color is obtained.

The reaction is as follows:

(v)
Aniline and N-methylaniline can be distinguished using the Carbylamine test.

Primary amines, on heating with chloroform and ethanolic potassium hydroxide, form foul-smelling isocyanides.

Aniline, being an aromatic primary amine, gives a positive carbylamine test. However, N-methylaniline, being a secondary amine does not.