You need to do a bit of reading. A good ID book should have a general explanation of grouping and why and then you can delve deeper on the internet. For example both the genus you mentioned are in the larger group Nembrothinae.
If you do most of your diving in the Indo-Pacific then the best current guide is 'Nudibranch & Sea Slug Identification: Indo-Pacific'. Although some of the names have changed it is still a great guide with a detailed introduction to the subject.