It really depends on the circumstances as there is a multitude of calculations that must occur for me to decide whether a flush is needed or not.
1st. I decide if Feces or urine is present.
2nd. If urine is only present I wil evaluate whether the tone is worth flushing or not. Generally speaking a darkish yellow would be subject to a flush, however, a pale yellow may be investigated further. If Feces is present then I automatically move to the third evaluation.
3rd. I analyize the water lever. If it is above normal I correctly assume that a flush is needed. However this may also be subject to further evaluation, as I may decide to not flush because in doing so, skids marks may be left.
4th. Depending on my mood, I will now evaluate if Im too lazy to scrub the skids marks (if i flush) or if I feel like being mischievous and therefore leave a massive amount of Feces in the toilet for someone else to discover.
Obviously I dont have enough time to go through all the other steps but hopefully you get the jest of it and perhaps I sorted out your problems. Pm me if you have any questions or need further tips or tricks.
All the best