There are several important tests for organic functional groups. As with all the tests for other types of compounds and groups there are plenty of alternatives and a simple and clear indication that gives reliable results for a specific group is important
Some of these tests are slightly more complex and so there are pages for some of the lengthier tests.
Functional groups tests
Add bromine water (orange in colour)
If a C=C double bond is present the liquid will turn colourless.
The yellow/orange solution forms a bright yellow / orange / red precipitate which when filtered off can be tested using melting point to determine an exact structure
See the Tollens’ test page
Although fairly weak acids, carboxylic acids can be detected using typical tests for acids.