How can I tell if a journal is reputable?
If you know your field, it is easy to know how reputable some journal is. You find papers in it by the best people in your field publishing the most important results there you are already familiar with.

On the other hand if you do not know your field yet, I would advise you to get to know it at the glance at least first. Familiarise yourself with the most important and fundamental results and findings and the most influential authors. You should identify these authors quickly by seeing them in many various papers cited again and again by different researchers. Then look where these authors published the papers to find out what journals are likely to be more reliable than others.

Moreover, if you know your field, you should be able to discern a value and validity of a paper quickly from the abstract.