If you do not know which printing you have, check the publication information on the reverse side of the title page.
For example, if you see:
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
then you have the first printing.
If you see:
Printed in the United States of America
10 9 8 7 6 5 4 3
then you have the third printing.
If you see:
Printed in the United States of America
10 9 8 7 6 5
then you have the fifth printing.