how to determine if a chemical has a certain element?

Answer 1

Aside from modern, high tech instruments, there are old fashioned tests for various elements.

For example, a flame test will turn colors with various elements. You can react with various solutions and see if a precipitate forms. For example, if a ppt. forms when you add chloride ions, you might have silver.

You can also test physical properties such as density, conductivity, melting, boiling points, etc.