How can I assess whether my power source is sufficient for the Raspberry Pi 3 I am running (including all peripherals)? Is there a tool which can be installed on the Pi to install this or will external hardware be required?
You cannot monitor power usage on the Pi, although as @joan mentioned you can detect if the Pi exceeded the capacity. The current drawn can vary widely with load so this is not reliable (although if it indicates consistent low voltage this can be regarded as definitive).
What you should do is add the current requirements of your peripherals and add ~700mA for the Pi. Unfortunately you cannot rely on the rated current capacity of most supplies. While I am sure they can safely supply the rated current they may not do so at voltage which the Pi regards as acceptable. The only way is to measure with an artificial load.