The phrase "are often suspicious" is correct and usable in written English.
It can be used to describe a general tendency or behavior of a group or individuals who frequently exhibit doubt or mistrust towards something or someone.
Example: "People who have been misled in the past are often suspicious of new information presented to them."
Alternatives: "tend to be wary" or "frequently express doubt".
"Critics are often suspicious of versatility.
Children are often suspicious of food that contains too many visible ingredients; it's better to go for simple dishes.
Raymond Perrier of Interbrand Newell and Sorrell (which, despite the Sorrell, is part of Omnicom) admits that firms are often suspicious of the advice given by their agencies.
They are often suspicious about employees who work out of sight, believing they will shirk or drift if not under constant supervision.
The poorly trained and only moderately organized militias are often suspicious of one another and are sometimes openly hostile, posing difficult problems for the transitional council now trying to govern the country.
Iraqis I've interviewed are often suspicious of U.S. intentions ("You're planning to steal our oil!"), but they're willing to give us a chance if we can stop rapes and get factories open.
China is often suspicious of that expectation and insistent on its own terms for any such role.
Aware of the precedents I list above, the left is often suspicious of those tiny few with a mainstream platform.
A next-door neighbor said he knew nothing about the owners, though he admitted there was often suspicious activity there late at night.
For policymakers like Rove, he provides an invaluable link to the Republican Party's conservative base, which is often suspicious of elected politicians and officeholders.
I'm often suspicious of novels with writers as central characters since there is always the chance that the author is engaged in a self-help exercise but Cercas's story of the young Spanish writer who takes a teaching job in the US and befriends an off-beat Vietnam vet with a dark secret is utterly absorbing from the opening page.
