You can jump on me for this post if you need Muerte. Please correct me if Im wrong.
I think in regards to things of ill nature, you would be able to tell easily if it was a spirit or a demon. In my opinion spirits are no where near as powerful as demons. Spirits would cause general annoyance and inconvenience. Ill wishing demons would cause havoc and at EXTREMES, bodily harm. Spirits can be easily pushed away (nice way of saying gotten rid of ) by prayer, blessings, spells, etc. Demons take quite a bit of work. The same type of things can be used but to a much more mindful extent.
If you gave more specifics you might get more helpful answers.