The ornamental design for diagnostic device, as shown and described.
FIG. 1 is a perspective view of a diagnostic device in accordance with my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a right side view thereof,
FIG. 5 is a left side view thereof; and,
FIG. 6 is a rear view thereof.
The broken lines in the inside of the diagnostic device and shown on the back are included for the purpose of illustrating portions of the diagnostic device that forms no part of the claimed design. Additionally, the cords are shown broken away in order to indicate that a specific length of the cords forms no part of the claimed design.