Has a photon or electron ever been observed in a state of superposition? Has subatomic particles ever been seen in a state of superposition or do we just detect information like qubits about the state of the particle? So is actual matter in superposition or is it just information about matter that's in a state of superposition?
 A: Perhaps I do not understand the question. When, for instance, a photon is observed in a state of circular polarization it is simultaneously in a superposition of linear polarization states. Every pure quantum state $\psi$ is always a coherent  superposition of other quantum states eigenstates of observables which are not defined in the state $\psi$. A quantum system stays always in a superposition of states as, in QM, mutually incompatible observables do exist.
A: 
Has subatomic particles ever been seen in a state of superposition 

as the other answers state one cannot label an individual particle until measurement of some of its variables. After measurement the information is a specific value of (x,y) or (p,E) at time t. 

or do we just detect information like qubits about the state of the particle?

Just checked what qubits are, but it does not seem to me to be coordinates or four momentum.

So is actual matter in superposition or is it just information about matter that's in a state of superposition?

Each individual particle has actual mass at detection, i.e. its energy and momentum define it uniquely as the measure of the four vector.
The theory of quantum mechanics predicts the accumulation of measurements as a probability distribution, thus one measurement gives one point in a probability distribution. In the case of the double slits interferences appear even when one photon at a time goes through.

So is actual matter in superposition or is it just information about matter that's in a state of superposition?

I see that qubits are defined as :

A qubit is a two-state quantum-mechanical system, such as the polarization of a single photon: here the two states are vertical polarization and horizontal polarization. In a classical system, a bit would have to be in one state or the other. However quantum mechanics allows the qubit to be in a superposition of both states at the same time, a property which is fundamental to quantum computing. 

With the mathematics of quantum mechanics this last question is unanswerable, because quantum mechanics only deals with probabilities. There have been and are still on going efforts to get the probabilities from an underlying real system , as with thermodynamics from statistical mechanics, unsuccessfully ( in the sense they are just for specific models, even when successful as in Bohm mechanics) .  At present the boundary conditions to the quantum mechanical problem determine the solutions where the probability distributions may show interferences, but individual particles appear whole when measured.
A: Yes, superpositions can be, and have been, observed using weak measurements.
There seems to be some debate about weak measurement and whether it is really observing a superposed state. This isn't my area so I can't comment. If you're interested in learning more there are a fair few questions about weak measurement on this site.
A: A superposition cannot be measured. Superposition is the state of a particle in the Copenhagen Interpretation of quantum mechanics before it is measured. Once you measure/observe it, it is no longer in a superposition.
