I only know of one way to really know - use a router that can log downloads & uploads by IP address ("IP Traffic client monitoring"). And it'd be helpful to assign the same IP address to each device with Static DHCP (assigns same IP by MAC address) or manually set IP addresses.
If your router doesn't log that info, you'll probably need to use custom firmware like OpenWrt, DD-WRT, Tomato... there's lots. Probably best to search by your router and see if any can work with it. And FYI, most are based on linux too, like Ubuntu is!
I've used TomatoUSB for WRT54G-family routers (specifically the "Toastman" version), most don't seem to have been updated lately, but the routers they work on can be 5-10 years old.
Here's a quote from the TomatoUSB page:
Tomato USB is an alternative Linux-based firmware for powering Broadcom-based ethernet routers. It is a modification of the famous Tomato firmware, with additional built-in support for USB port, wireless-N mode support, support for several newer router models, and various enhancements.
And a little info about Toastman's version: (That's his avatar)
Using the QOS I was able to prevent uncontrolled P2P users bring down the router every few hours...
it allows the use of Tomato with up to 250 users. I use it in environments where we have no control at all over those users' PC's or what applications they may be using. Therefore, the QOS had to be very effective so that they could all share the same internet gateway without problems.
[FYI, "Quality of service (QoS) is the overall performance of a telephony or computer network, particularly the performance seen by the users of the network" - Wikipedia]
The QOS can be used to limit the network usage of certain traffic (like P2P) so the network "feels faster" for other uses too. Here's a guide from Toastman about it.
Here's a good looking article about "Using the Tomato Firmware to Monitor Bandwidth by IP Address" on an Asus RT-N16 router, and a son flat-out lying to his father ("I don't know what uTorrent is, or why my computer's uploading 9GB/day, honest!") and a (cropped) screenshot of the "IP Traffic Daily History"