It sounds like you're really asking about how to measure in a given Pauli basis. (I'm assuming for simplicity that you are working with only a single qubit, but the below is generalizable to multiple qubits as well.)
Q# has a Measure operation which performs a measurement in a given Pauli basis. For example, if you have a Qubit
object q
in some state $|\psi\rangle$ and you want to measure it in the $X$ basis, you can write:
let result = Measure([PauliX], [q]);
Here result
will be either Zero
or One
. If you want to then estimate the expectation value of the measurement, you would need to repeat your full operation (including whatever you did to prepare your qubit in the state $|\psi\rangle$) many times and take an average of the measurement results.