Methyl amine when reacts with $\mathrm{N}\mathrm{a}\mathrm{N}{\mathrm{O}}_2$ and $\text{HCl}$ gives ${\mathrm{N}}_2$ , while aniline gives diazonium salt. Diazonium salt so formed give red colour dye with alkaline $\mathrm{\beta }$ naphthol. Methyl amine however does not produce dye. So these two can be distinguished by using these reagents.