Test the waters. Merriam-Webster defines it as “to make a preliminary test or survey (as of reaction or interest) before embarking on a course of action.”
Trial balloon. This is “a project or scheme tentatively announced in order to test public opinion.” The term is a translation of the French ballon d’essai.
Dip a toe in the water. This means “to try an activity briefly to see if one likes it.”
Kick the tires. This means to test something before buying it. The phrase is American and comes from people shopping for cars.