The best indicator of a person's level of optimism is his or her:

The best indicator of a person's level of optimism is his or her attributional style.

Attributional style is a term which refers to how you explain the causes of some events. For example, something bad happens - and based on whether you find the cause to be your own fault (negative cause) or tend to see a brighter side in it (positive), your level of optimism can be measured, which is related to your attributional style.