Use your voltmeter- measure voltage at the battery terminals. With the outboard off, a charged battery will show a voltage of slightly higher than 12 volts. With the outboard running at idle, you should see a slight increase in voltage. As you increase rpms, you should see an increase in voltage, up to almost 15 volts. Any readings out of these specs indicates a problem with the charge system. If you feel that your battery is self-discharging at an abnormally high rate, have the battery load tested. As a rule, most marine flooded batteries begin to show high rates of self-discharge after 3 - 5 years (this is dependent on battery quality and usage, of course).
