They feel warm. I can easily tell when my baby's temperature is slightly off.

I can see a difference in his actions and his facial reactions changes

They become super fussy and of course I can tell by touch.

When my son is not bubbly, cries a lot and feeling warm.

Change in behaviour is already a telltale sign...

Yes luckily i am quite sensitive to such things

The cry is different is you listen closely

They become really grumpy and irritable.

Just check frequently with thermometer

Sometimes but i always need to check.