Is it a geometric sequence or not?
Jelly, 4 bytes
Ṣ÷ƝE

Try it online!
Thanks to fireflame241 for pointing out a mistake
How it works
Ṣ÷ƝE - Main monadic link, takes an array as input
     - e.g                            A = [2, 8, 4] or   A = [2, 6, 14]
Ṣ    - Sort                               [2, 4, 8]          [2, 6, 14]
  Ɲ  - Over overlapping pairs [x, y]...   [[2, 4], [4, 8]]   [[2, 6], [6, 14]]
 ÷   - ...divide x by y?                  [0.5, 0.5]         [0.33, 0.43]
   E - Is the list all the same?          1                  0