Looks like you are trying to program some step motor controller or something similar.
Usually in such controllers you should wait for controller response to verify that command was processed properly.
It looks like that your design of code is very bad. Move everything related with this controller to separate class, which has set of slots, something like: starRotateLeftWithSpeed(double)
. Code will be cleaner and it will be easy to use thread if you decide to use methods like waitForBytesWritten
proposed in another answer.
Definitely you should read controller manual more carefully.