The model designation alone gives such listings away. The number 145, colloquially known as the Chopin, denotes the fountain pen of the Meisterstück Classique line. A rollerball with that number never existed; within the Meisterstück line Montblanc lists rollerballs in the Classique and LeGrand sizes under numbers of their own. Checking whether the combination of model number and writing system on offer ever existed at all will expose many fakes from the description alone.
The packaging and papers are telling too. A black star on a white box does not match Montblanc's presentation, and the Service Guide enclosed with fakes frequently contains a spelling error, such as “writing in struments” in place of “writing instruments”. Another common feature is a star in the cap top cut too sharply and too angularly, whereas the original shows softly and evenly rounded points. Where several such oddities come together, the pen is best left alone.