Do I need to show a return ticket upon arrival at Bangkok airport with a one-way ticket?

with a german passport you're probably not gonna be asked

@Kadin *****

if you manage to check in and board your plane you should be fine, you won't have a problem once you get here

you are comin' in on a 30 day visa exempt entry stamp then.

I'd say IF you manage to check in with your airline without proof of onward travel coming here without a valid visa for thailand in your passport you are highly unlikely to be asked for proof of onward travel once you land here.

It's the airline you're comin' here on that could cause you the problem when you check in.