Grid tie inverters and islanding
Answer from a non-electronics person who's been reading up on this over the last few days. Depends on standards and things (VDE-AR-N 4105 being a common one), but here is a non-exhaustive list:


Voltage change
Frequency Change
Rate of Change of Frequency (RoCoF, dF/dt)
Vector shift


Most of them makes intuitive sense I think, at least if your generator is mechanical in nature. A change in load usually causes your engine/power source to slow down, causing a drop in voltage, a change in frequency, and a vector shift. The vector shift is when a momentary slow-down causes a cycle to take sligthy longer than the previous cycle, so for example on a 50hz grid where a cycle is supposed to take 0.02 seconds, it might take 0.22 seconds for just two cycles before returning to normal, but that might be enough of an indication that islanding has occured.