There are two useful sections on status.lemm.ee for this - firstly, there is an automated check for federation with all other instances on the bottom of the page, and everything there being red is a definite sign that something is wrong with lemm.ee itself. Secondly, near the top of that page, I will always write a status message manually when I discover & start work on any issues. This second part can have a bit of a delay, as it requires manual input from myself, but I have updated it every time we had any issues so far.