They have a pH less than 7, see pH.
They will turn blue litmus paper red.

Hydrochloric acid and sulfuric acid will react with

1) Any alkali or base, see neutralisation.

2) Any metal above hydrogen in the reactivity series.
The metal will fizz, giving off hydrogen gas,
and leaving the metal salt in solution.
It is not safe to put a metal into an acid which is
above magnesium in the reactivity series.

Any chloride or sulfate can be safely made
by reacting the appropriate metal
(from lead to magnesium in the reactivity series)
with hydrochloric acid to make the chloride
or sulfuric acid to make the sulfate.

3) Any metal carbonate or metal hydrogen carbonate.
The metal carbonate or metal hydrogen carbonate will bubble
giving off carbon dioxide gas, leaving the metal salt and water.

Any chloride or sulfate can be made
by reacting the appropriate metal carbonate or hydrogen carbonate
with hydrochloric acid to make the chloride
or sulfuric acid to make the sulfate (see examples).