How to tell whether an optical spectrum is featureless or just with a low S/N? A featureless spectrum and a spectrum low S/N are very similar. 
Is there a way to tell the difference between them?
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You would need to do statistical tests. i.e. Is the data you have (given the known noise properties) consistent with a featureless spectrum? If not, then it has features. Of course, the lower the signal-to-noise or the weaker the features might be, then the harder it would be to rule out a featureless continuum.