how can you tell if ionisation, excitation or nothing is occuring?

ionization is just excitation to the full amount, the amount that causes the electron to break free.

The thing about electrons is that they don't just fill any old energy level, they fill very specific levels, more like going up stairs rather than walking up a ramp. So, to make an electron go up an energy level, it has to be hit by enough energy to make it all the way up to the next step. if the amount of energy is too low, it won't get the electron up. sort of like being too weak to throw a ball onto the roof of the house.

The thing about the "stairs" though, is that they are not all the same height, both within an atom and between atoms of different elements. Each element has its own particular "staircase" of energy levels, so each element can absorb very specific energies equal to the amount needed to kick an electron up one or several levels. since the stair heights are unique to an element, the amount of energy that is used will be unique to that element, so you can actually figure out the element by how much energy it absorbs to excite an electron.

To "know" whether a given packet of energy will excite, fail to excite, or ionize, you need to look at tables that tell the energy levels that are common for the element. those energy levels have been figured out by throwing energy at atoms and seeing what energies they absorb (that they use to excite).

I don't think you can know, unless it has been done before (someone already tested and wrote down the results) whether an energy will be enough to make an excitation or an ionization occur.

The rule is, if the photon (energy packet) is big enough to make an electron jump up one or more stairs, that is precisely what will happen when the energy packet interacts with the electrons. It will use the largest amount of the given energy that it can, no more, no less. If it isn't enough to kick up even one stair, it won't happen.