Separate the craft from the software
Learning a design application teaches you to execute an idea. Learning composition, hierarchy and typography teaches you which idea to execute. People who learn only the software tend to plateau at competent-looking work that does not communicate, and the fix is usually a course with almost no software in it at all.
Look for feedback, and be honest about its absence
Design improves through critique more than through instruction, and most self-paced courses cannot offer it. That is not a flaw so long as you know it: a self-paced course can teach you principles and technique, and you will still need somewhere to have work looked at. Where a course includes review, mentoring or a community, the listing says so.
Check what you finish with
A course that ends with finished pieces is worth more to a designer than one that ends with a certificate, because hiring in this field runs on portfolios. Where a provider describes projects or briefs, the summary records it. A course with no output at all is an orientation rather than training.
Check what software you will need
Some design courses run on free or browser-based tools, and some assume a paid subscription that costs more than the course. Where a provider states the requirement we record it. If the course is published by the company that makes the tool, expect the tool to be assumed.
Know which design you mean
Brand identity, editorial layout, illustration and interface design share vocabulary and very little else in daily practice. Course titles are often looser than the content. Read the syllabus for what is actually produced, and if you want screens rather than print, the product and interface material is filed separately in this directory.