One way to distinguish between the two is by placing your fingers on your throat.
When making the voiced sound /ð/, you should feel a vibration in your throat, while the voiceless sound /θ/ will not produce any vibration.
Another way is to pay attention to the position of your tongue.
at the beginning of functional words th is voiced.
Ex. in the article the and in the words this, that, these, those, then, than, they, them, their, there, therefore. at the beginning of most other words, th is unvoiced: thunder, theatre, think, etc.
The TH sound is in the words the, three, breath, and mother, and it is represented in the IPA (International Phonetic Alphabet) by these symbols: /θ/ for the voiceless TH sound, and /ð/ for the voiced TH sound (we'll talk about voicing a little later).