Your boat's Manual will have detailed directions on how to empty the tank.
Don't forget that most fuel docks can pump you empty. Open the access hole on your port walkway, stick the nozzle in and have them turn the machine on. When the clear hose shows nothing left passing through, have them shut off the machine and then screw the access port cover closed......
Yes, I always shut the discharge seacock closed after dumping the holding tank.
On mine, I can hear the macerator running and when I hear it more loudly "groaning" I know that it's then empty. If you want to really do the job right, flush a few toilet fulls of fresh water with a little of that blue deodarant in it into the holding tank, then flush it empty again and you're good to go......
My new macerator is a bit quieter than the old one so in order to "hear" it, I need to turn down the stereo, close the cabin up and maybe even shut the engines down.
A word the wise, anytime that you visit your boat, at least hit the discharge button for just one second...... just to keep it limbered up. :goodjob