How to differentiate center-tapped power transformer from a non center-tapped one?
A center-tapped transformer has three connections on the secondary, where a non-center-tapped transformer has only two.  Transformer datasheets should indicate this by showing a schematic symbol of the transformer.

The output voltage of a transformer is always given as the RMS value.  The peak voltage is about 1.4 times the RMS voltage, and after a bridge rectifier and filtering the DC voltage will be 1.4 volts less than the peak-to-peak voltage. 

A 30 volt center-tapped transformer with a bridge rectifier will produce about 40 volts DC.  If you use the center tap as the common, or ground, terminal, that gives you +/- 20 volts.