Not necessarily. It is possible to have dangerously elevated blood glucose without ketones, especially in people with Type 2 diabetes who are dehydrated. This is called hyperosmolar hyperglycemic state, and its symptoms include excessive thirst, hallucinations, sensory loss, rapid eye movement, paralysis on one side of the body, and seizure. It may be mistaken for a stroke.
Learn more about hyperosmolar hyperglycemic state here.
Check for ketones if your blood glucose level is over 250 mg/dl twice in a row, or even only once if you intend to exercise soon.
Learn more about ketones here.