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This is worth a little research before you conclude anything, because Thai descent law is unusually forgiving about distance and unusually strict about breaks - so the answer depends on facts rather than on how Thai the family feels.
What you are looking for is one thing: whether a parent of yours held Thai nationality at the moment you were born. Not a grandparent, not a great-grandparent - a parent. Section 7(1) of the Nationality Act passes nationality from parent to child, in or outside Thailand, with no generational limit and no rule against a citizen by descent passing it on further. So an ancestor further back matters only through an unbroken line of parents reaching you, and the question always comes back to the generation immediately above you.
The trap to avoid: treating documents as the test. Thai nationality by birth is automatic and is not created by registration, so a parent can be Thai and hold no Thai papers at all. A family that lost touch with Thailand two generations ago may still have a living, unregistered Thai citizen in it.
What to look for, in rough order of usefulness. Your parents' birth certificates, and whether either was born in Thailand or registered at a Thai embassy. Any Thai identity card, passport or house registration entry - the tabien baan - for a parent or grandparent. Your own foreign birth certificate, which may record a parent's nationality. Family memory about whether an ancestor left Thailand, and when, because someone who renounced Thai nationality on naturalising elsewhere broke the chain at that point for children born afterwards.
What happens next: if what you find points to a Thai parent, come back to this, because the follow-on questions - which parent, whether your parents were married, what year you were born - are what decide it, and the claim has no age limit or deadline. A Thai embassy or consulate for the country you live in can tell you what evidence they would accept before you commit to gathering it.