Quantum computers can be simulated classically, in a timeframe exponentially related to the number of qubits. So as long as the quantum computer is not too big

Quantum computers can be simulated classically, in a timeframe exponentially related to the number of qubits. So as long as the quantum computer is not too big you can compare the results to an exact classical simulation and see if it works. The current record for classical simulation is 56 qubits (https://arxiv.org/abs/1710.05867 https://arxiv.org/abs/1710.05867). This is an active area of research, since we want to be able to check the early-generation quantum computers carefully.