How can I see how much bandwidth each device is using on WiFi?

Check the router's admin page or its companion app for a device list, traffic monitor, or bandwidth statistics. Some routers show live upload and download rates for each device, while others also track totals over time. Some don't offer per-device figures at all.

You'll need access to the router to see network-wide usage. Match the listed device names or addresses to the actual devices before drawing conclusions, since the names aren't always obvious.

If the goal is to stop lag rather than identify a culprit, look for QoS or traffic control in the router settings. Depending on the router, you may be able to prioritize a device or set limits so one connection can't crowd out everything else.

Ask whoever manages the router before changing anything. The available controls vary, and an overly restrictive limit can make another person's connection frustrating to use.

Don't assume a particular person is maxing out the connection just because the timing lines up. High-resolution streaming, downloads, and file sharing can all generate substantial traffic, but WiFi interference or another network issue could also be involved.

Compare the router's live traffic readings with the times the slowdown happens. If the traffic isn't especially high, testing closer to the router or over a wired connection may help narrow down whether WiFi is the problem.