1 to doubt (sth)

2 to be skeptical of

3 to have one's doubts

4 to harbor suspicions

5 to suspect that

1 to call into question

2 to question (truth or validity)

1 suspicious

2 dubious

1 mistrustful

2 suspicious

3 paranoid

1 suspicion

2 to have suspicions

1 to believe firmly without any doubt (idiom)

2 absolute certainty about sth

1 cannot be doubted (idiom)

1 cannot be doubted (idiom)