History, exam, eng. Usually based on a good history (discussing the details of your symptoms), physical exam (including hearing, looking into ears, neurologic exam, and vestibular exam), and perhaps formal hearing (audiometry) and balance center testing (vng/eng). Confirming diagnosis may often depend on seeing how you do with trying some medications as well. Imaging (ct, mri) often done to avoid missing other causes.
Clinical diagnosis. Meniere's is a clinical diagnosis, with a constellation of symptoms characterized by fluctuating hearing loss, ear fullness, tinnitus that fluctuates and episodic vertigo/spinning that lasts for 10 minutes to hours. Other conditions are ruled out like vestibular migraine or acoustic neuroma. Tests include mri, hearing test. Sometimes ecog or eng are ordered by these do not "make" the diagnosis.