Using the top navigation bar, click the feature ‘User Look-Up.’ This is where you can get a live stream of events tracked in the last 24h.
Select a specific user, and you’ll see the user profile, user ID, amplitude ID, and all the device IDs that Amplitude has seen, including other information (i.e.: demographics, SDKs, when we first saw the user, live event stream, etc.)
If you see a User ID, it's because it has already been set. If the value is null, the User ID has not been set.
There’s also a raw version that you can access for debugging purposes by toggling to ‘Raw.