First sign is that a note gets stuck sending, then "connecting" and we're back to logging in again.
Also, often I'll see something about not happening within 5 seconds and it's an error.
To be fair this is usually (but not always) when I'm stress testing Nostr on a sub 1MB mobile connection from my boat out in the sticks.
Nostrgram is the best client for properly sized screens
Posted from my boat, with Snort because I've just been dumped out of Nostrgram :-)