General Suspicion:
* Dubious: Raises doubts or suspicions.
* Suspicious: Makes you think something is wrong or dishonest.
* Questionable: Open to doubt or uncertainty.
* Shady: Dishonest or illegal.
* Dodgy: Unreliable or risky.
* Shifty: Looking or acting as if you are hiding something.
* Evasive: Avoiding giving direct answers.
More specific words:
* Deceptive: Intentionally misleading.
* Tricky: Designed to deceive or mislead.
* Underhanded: Dishonest or sneaky.
* Cunning: Skilled at deceiving others.
* Machiavellian: Cunning and unscrupulous, especially in politics.
Informal words:
* Phony: Fake or insincere.
* Bogus: Fake or not genuine.
* Crooked: Dishonest or corrupt.
The best word will depend on the specific situation and the level of formality you need.