# Testing fit of probability distribution

If I have fitted training data to a probability distribution, e.g. a poisson distribution, how can I test this fit on some test data?

To fit the poisson distribution I am using R's fitdistrplus package that using MLE for determining the optimal coefficients of a given distribution. Therefore, I have the estimated $\lambda$ for a poisson distribution based on my training data but I am not sure how to test this on some unseen test data.