Both ड and ड़ are retroflex sounds, but while the former is a (voiced) plosive, the latter is a flap. If you have already learnt the Hindi "r" (र), which is an alveolar flap, and know that the English "d" is a (voiced) alveolar plosive, then, ड़ is to ड what र is to the English "d".
If that didn't help, there is another way to know the difference between them and that is to listen to their pronunciations. The sounds are distinct enough to be easily discernable. Listen to the audio on the Wiki pages: ड and ड़, and here on Google Translate.