How to align the input end of a right-angle planetary gearbox with the motor shaft?

Hello everyone, the equipment space is very tight. The servo motor is directly plugged into the input end of the right-angle planetary gearbox (https://www.oyostepper.com/category-192-b0-Right-Angle-Planetary-Gearbox.html), with no space for a coupling.

I'm worried that the motor shaft and the gearbox input hole are not concentric, which will damage the bearings over time. However, there's no space to fit a dial indicator, and visual alignment is unreliable.

Are there any simple methods to roughly confirm the alignment? For example, using a feeler gauge or a coloring method? Or, with this type of direct-insertion structure, as long as the flange mounting surface is machined to a sufficient precision, there's generally no need to worry?

I hope an expert can help me.