Work out which job the course is about
Product management, user research and interface design are three roles that collaborate and are frequently sold under one heading. A course on prioritisation and stakeholders will not teach you to run a usability test, and a course on prototyping will not teach you to decide what is worth building. Read the syllabus for the artefacts produced.
Value the research, not just the screens
Interface work is the visible part and the easiest to teach with screenshots. Research, which is how anyone knows the interface is wrong, is harder to teach and easier to skip. A course that goes straight to layouts is teaching craft without the evidence that directs it.
Keep tools in proportion
Design and prototyping tools change and are learnable in days. Method, research practice and judgement take much longer and stay useful. A course that is largely a tool tutorial is fine when that is what you want, and a poor substitute when it is not. Check whether a paid tier is needed to complete the exercises.
Notice the size of organisation it assumes
Much of the well-known material in this field comes from large technology companies and assumes their structure: dedicated researchers, several teams, a real backlog. If you are the only person doing this work, some of the process will not transfer, and knowing that in advance saves a lot of frustration.
Check what evidence you finish with
Hiring in this field usually runs on case studies that explain decisions, not on finished visuals. Courses that make you document reasoning, not only produce screens, are the ones that leave you with something to show. Where a provider describes projects, the listing records that.