15, 18, 21, 24

How to tell if it is arithmetic.

1. Subtract the 1st term from the 2nd term.  Notice what you get. 18-15 = 3
2. Subtract the 2nd term from the 3rd term.  Notice what you get. 21-18 = 3
3. Subtract the 3rd term from the 4th term.  Notice what you get. 24-21 = 3

Since we got the same difference 3 each time we subtracted a term from the
next one, the difference 3 was in common.  So we call 3 a "common difference"
and the sequence is an arithmetic sequence.

Edwin