A bubble chamber often contains liquid hydrogen. Charged particles entering the chamber would interact with the electrons transferring some of their energy via the Coulomb force. This initiated boiling and thus led to bubbles being formed. The electrons of the hydrogen atoms thus acted as the detectors within the chamber. Only charged particles cause tracks in bubble chambers, as neutral particles will not interact via the Coulomb force.
However, we do not just want to see a beam of particles passing through a chamber. We want to see what happens when nuclear particles interact with each other. The beams of particles interact with the protons of the liquid hydrogen in the bubble chamber and so the bubble chamber contains both the detector particles and the target particles.
How are the particles moving?
The beams of particles are parallel to begin with. Some may then undergo collisions and the path will change. So you need to look for beams that do not go directly from one side of the picture to another.
What charge is the original beam?
If we are not told what the original beam is we may sometimes be able to tell what its charge is from just looking at the picture.
A visible beam has to be made up of charged particles. These will be bent within a magnetic field and so the direction of curvature will tell you the charge of the particle. Often a beam will collide with an electron, knocking it on with more energy than usual. This then leaves a track of its own within the detector that is quite distinctive.
There are often a few examples of this appearing on a picture and this shows the way that the negative charges are bent in the magnetic field. You can then compare this with the direction of curvature of the particle beam to determine the charge of those particles. Using Fleming’s left hand rule you can also deduce the direction of the magnetic field in or out of the picture.
Knowing the direction of the magnetic field you can then find the charges of all particles.