How do you know when a syllable is stressed or unstressed? Can any syllable be stressed/unstressed?

I have read the sticky, but so far it seems to have only given me examples of stressed and unstressed syllables.
[U]        | |                     [/U]
[U]        |/     .-.              [/U]
[U]       /|_     `-’       |      [/U]
[U]      //| \      |       |      [/U]
[U]     | \|_ |     |     .-|      [/U]
      *-|-*    (_)     `-’
well, a very common meter is the eight-bar subdivided into two-and four-bar units.

DUM da DUM da DUM da DUM 4 stresses
DUM da DUM da DUM 3 stresses
DUM da DUM da DUM da DUM 4 stresses
DUM da DUM da DUM 3 stresses

Mary had a little lamb
its fleece was white as snow
and every where that Mary went
the lamb was sure to go

i hope that all made sense. hopefully you can use that example in other situations for more complex meters
you can 'stress' (i would call it 'accent') any beat by putting this symbol above or below the note: >. if there arent any then its down to the players interpretation of the peice but its normally assumed that 1 and 3 are the strongest beats in 4/4 (1 being slightly stronger than 3), 1 is strongest in 3/4, 1 and 4 in 6/8 and so on. if you play a piece in any meter though you will feel which notes feel strongest to you (start by playing all of the notes with the same dynamic) and then your own interpretation becomes important.
i only wear black until they invent a darker colour

everyone is sh*t....were just all at different levels of sh*t
^he's talking about words.

basically, speak the words aloud. you can hear for yourself where the stress naturally falls.
my name is matt. you can call me that if you like.