Is it possible to detect and diagnose Crohn's when the patient doesn't have any symptoms? For example, the patient had symptoms for over a year, which are not longer present.
You can screen for occult bowel inflammation using the stool test fecal calprotectin. However, a normal test doesn't exclude inflammatory bowel disease and a positive test is highly suggestive. Capsule endoscopy is a procedure when a person swallows a camera in a pill that allows pictures to be taken of the bowel. This can also aid in the diagnosis of inflammatory bowel disease.
The first line of investigation for diagnosis of Crohn's disease is endoscopy (gastroduodenoscopy and ileocolonoscopy). Endoscopic remission (healing of mucosal lesions) does not necessarily correlate with clinical remission (disappearance of symptoms), and endoscopy would therefore still be indicated as a first-line diagnostic test if there is a very high suspicion for the disease based on previous symptoms.
For example, take a 21 year old man of Ashkenazi Jewish ancestry, with two sisters having Crohn's disease, who has a resolved history of bloody diarrhea 1 year ago, and is found to be anemic and B12 deficient on blood tests. This patient would warrant endoscopy even if asymptomatic at the time of presentation.
As the other answer notes, fecal markers of inflammation (fecal calprotectin) and serologic markers of inflammation (ESR, CRP) could be used. In this situation, their utility would be as a "rule out" test if the suspicion for the disease is rather low and the goal is to avoid having to perform an endoscopy. If those markers are negative, and there are no symptoms, it may be appropriate to defer performing endoscopy until symptoms re-appear.
Finally, capsule enteroscopy, CT enterography and MRI enterography are non-invasive tests that can be used to evaluate the small bowel, which may show mucosal disease despite the absence of symptoms. CT and MRI can also delineate other complications of Crohn's disease such as fistula, abscess, or bowel perforation, though these would definitely present with symptoms.