Before calling the Core.Execute method as mentioned in Marco's question you referred to, you can check with JAVA code if the microflow exists with the following code:
boolean exists = ModelerAPI.getMicroflows().containsKey("YourMicroflowName");
After this the variable exists will be true if the microflow is found (exist), otherwise it would be false and you can handle it in the way you want.
Note: If you call the Core.Execute method with a non-existing microflow name the Core will throw an exception (witch can be catched if you want). This isn't relevant if you'r using the check I've described above.
Will the java action compile if you are refering to a non exisitng Microflow?