To believe something means to accept a claim as true. To disbelieve something means to reject a claim as false. But to doubt means to question whether a claim is true or false.
Belief is a decision to accept a claim. Disbelief is a decision to reject a claim as false. But doubt means to suspend judgement about the claim until further evidence clarifies the matter
Similar But Not the Same
Doubt and disbelief can look similar. They both refuse to accept the claim. But they are not the same. Doubt is indecision, disbelief is a decision. Douby is inaction, disbelief is action. It rejects the claim as false and then acts as if the claim is false. Doubt does not act on the truth or falseness of the claim
Know Them by Their Fruit
This is why Jesus told us to judge people by their fruit. By fruit he is talking about their daily conduct, both in words and deeds. These will indicate what a person really believes.
We can also use the same method to understand our own beliefs. No one has deceived us so often as ourselves. Examine yourself, what do your words and actions truly say about what you really believe.