Do this carefully. Go into the format menu and it will show you a bar graph of how much card space is left. Just make sure you are careful or you will format your card, although it takes a turn of the dial and 2 button presses to do this.
I typically use my h4n to capture audio straight into my 5D. What I do is run a splitter right out of the zoom. One goes to my headphones for monitoring and the other goes through an inline volume control into my 5D. The volume control allows you to monitor at a reasonable level while feeding a strong signal into the 5D. The real key here is to turn the input into the 5D very low (I use 5 clicks above the bottom) to defeat the AGC. Set the inline volume attenuator using the audio meter and you are good to go.