The ornamental design for an indicatingdevice for a game of darts, as shown and described.
FIG. 1 is a front perspective view from above an indicating device for a game of darts showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines shown in the figures are for the purpose of illustrating portions of the indicating device for a game of darts which form no part of the claimed design.