What should raise red flags to detect fabricated data
There is only one reliable way to do it, which is to try to replicate their results.

The unreliable, but not completely useless way, is to see if the numbers fit Benford's Law.
Benford's Law describes the distribution of the first digit of many very diverse data sets. This is the distribution:


(public domain chart from Wikipedia)

Andreas Diekmann describes this further, in Not the First Digit! Using Benford's Law to Detect Fraudulent Scientific Data, a paper in the Journal of Applied Statistics from 2007