How to distinguish between CsCl and NH4Cl by a chemical test?
Correct you can distinguish them by their reaction with a base. You should use a base, such as $\ce{NaOH}$. $\ce{Cs^+}$ wouldn't react with $\ce{NaOH}$, while $\ce{NH4+}$ will undergo reaction $$\ce{NH4+ + OH- -&gt; NH3 + H2O}$$ $\ce{NH3}$ is a gas and can be detected with a moist litmus paper.

If you will, you can actually detect $\ce{Cs+}$ using a procedure from this paper:

"Selective Detection of Cs+ in Water Solutions via One-Step Formation of a New Type of Struvite-Like Phosphate". However, this is an exotic reaction (in practice you would use the flame color test) and you don't have to do it since you only need to distinguish samples.