How can I tell if I have primer-dimers in my PCR reaction?
FAQ ID -552

In quantitative (real-time) PCR, primer-dimers will appear as a peak with a Tm lower than the Tm of the specific product. This peak will also appear in the no-template control (NTC). In non-quantitative endpoint PCR, primer dimer will appear as a more or less faint smear on an agarose gel, below the product band of interest.