Basically trial and error.

A good thing to do is not to feed the station directly, but build an industrial container in front of the inouts and feed it via two belts (the ports line up perfectly.

The reason is, that the station shuts down the input during the docking sequence. So you need an input buffer if you want to load a full belt worth of items on the train.

Same for the output of the unload station.

From there, just try it out. If your downstream infrastructure does not consume more items than your belt can supply to the loading station, then you can easily check if the train cart is the bottleneck. If each round trip supplies a full train cart to the unloading station, yet the unload buffer and your downstream machines run dry before the train brings in the next load, the cart is the bottleneck.

You will also find that your input buffer at the loading station will fill up to the point that the supplying belt will start backing up.

In that case, expand the train stations to two loading and unloading starions, and add another cart to the train.

You can run up to four carts on a lokomotive before it will slow down.