You can get a list of all valid dimensions and metrics from the metadata API , however there is currently no way to check if the dimensions and metrics are compatible with each other. The only way to do this is to send a request or, as you said, check the sizes and indicators .
I have asked for this quite often over the years. They say they would like to add something like this at some point. However, I think its probably a low priority list.
Answer: There is no way to programmatically check if the measurements and measures are valid together before sending the request.