Say I have a large soft surface such as a jacket and I want to register it getting hit by a paintball from a paintball gun, how would I do that? What type of sensors could I use?
a piezoelectric sensor will create an electrical signal when hit. They are used in many different types of devices and would most certainly register a paintable hitting a jacket.
Not going to be easy, i don't think an accelerometer is going to cut it.
I think the easiest answer would be to modify the jacket (or make an under shirt) which is pressure sensitive.
You could do this by using 2 layers of conductive fabric and a sparse semi insulator in between and carefully sewing it all together. Setup a circuit to measure the resistance between the 2 conductive fabric layers and when something compresses them (such as a paintball) the resistance should drop.
Alternatively, if a paintball is really your use case, you could use a microcontroller with a little microphone somewhere on the jacket and process the audio looking for the sound of the impact. More math work, less sewing.
How about piezo film sheet eg http://www.meas-spec.com/product/t_product.aspx?id=2488 ?