Does each of these examples have a subject? Yes. Does each have a verb? Yes. So what makes the thought incomplete?? It’s the first word (Because, After, When, Since, If). These words belong to a special class of words called subordinators or subordinating conjunctions. If you know something about subordinating conjunctions, you can probably eliminate 90% of your fragments.
First, you need to know that subordinating conjunctions do three things:
join two sentences together
make one of the sentences dependent on the other for a complete thought (make one a dependent clause)
indicate a logical relationship