I am using a timer node and I need to see if it is running or not. I tried $Timer.is_active() but it says it is a non-existent function.

I have not tried it myself, but the documentation says the time_left property can be used to detect if a timer is active or not. Apparently time_left will be zero when the timer is finished, so something like this should work:

if ($Timer.time_left > 0):
	print ("Timer is active")
else:
	print ("Timer is no longer active")

The timer will also send a timeout signal if you set it up.

This probably isn't interesting to anyone, but I am doing a turn based game so I am playing around with this. You can use coroutines to wait till a function is finished and wait for a timer, which I am doing in my game:

func _ready():
	yield(wait(2),"completed")
	print("time is up")

func wait(amt):
	$Timer.start(amt)
	yield($Timer,"timeout")

I don't know how you get it to format. I pressed the code button.