There are three ways for working out if a sound is voiced or unvoiced:

1. Put your hands over your ears & say the sounds - you'll hear the voiced sounds.
2. Put your hand on your throat while saying the sounds - you'll feel a vibration for the voiced sounds.
3. Put a piece of paper in front of your mouth when saying the sounds - the paper will move when saying the unvoiced sounds.

http://www.developingteachers.com/phonology/sounds.htm

To know this helps learners with the pronunciation of sounds, & also helps teachers with correction. This un/voiced distinction also helps us to work out a couple of rules. Have a look at the following examples & work out the rule:

For plurals & third person singular verbs in the present simple:

plurals ending in
shirts
books
plates

plurals ending in
eggs
days
trains

plurals ending in
buses
watches
kisses

So why are do the above words end in those sounds?
So now think of some third person singular present simple verbs (goes, visits..) & see if the same rules apply.

For past tense regular verb endings:

verbs ending in
watched
booked
talked

verbs ending in
played
followed
cleaned

verbs ending in
wanted
highlighted
studied

And again, why do the above words end in those sounds?

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Answers to the plurals & third person singular verbs in the present simple:

shirts
The last sound before the end one is unvoiced so it is followed by

eggs
The last sound before the end one is voiced so it is followed by

buses
The last sound before the end one is a sibilant sound
,,,,,

Answers to the past tense regular endings:

watched
The last sound before the end one is unvoiced so it is followed by

played
The last sound before the end one is unvoiced so it is followed by

wanted
The last sound before the end one is or so it is followed by

So now we know the rules, what do we do with them? I know some teachers who introduce these rules to their students. Ok, the /t/ & /d/ endings for the past tense verbs followed by /Id/ is an easy rule to remember but I cannot see the point of introducing the rest to our learners. Are they really going to think about them before they speak? Are they going to go through the process of deciding if the penultimate sound is un/voiced & then remember the rule & then apply it. Highly unlikely.
These are areas where the students have to develop a feel for whether they sound right or not, develop their intuition. However they are rules that we as teachers need to know, as we need to know what is happening & they help us with correcting.