Yes, this is most likely thrips. The best way to test for them is to treat for them. If the treatment works and the bud drop slows or stops then this proves that thrips were the cause. In southern CA you can count on thrips showing up at least once per summer, and sometimes 2-3 times. If you do not treat for them they will become established and cause most hibiscus buds to fail to open.
See our thrips page for suggestions on how to control them.
http://www.hiddenvalleyhibiscus.com/care/thrips.htmCharlie