Baron is right on the money with the markings, the badge colour denotes company and on the other shoulderpad should be a flame symbol, unless they are first company where they have the vet cross apparently.
Of course you can just use the codex markings anyway, there has been numerous changes to this in sallie history.
As for marking sergeants i've seen a couple of ideas:
Flame honours - this is a flame symbol on the kneepad or leg armour. Looks pretty good as long as it's noticeable
Gold helmets - i don't really like this look but i've seen a lot of people use it
When i painted my command squad i decided to paint their helmets part green & part black, like so:
Which i think would make a nice sergeant marking.
If i had actually planned my army properly, instead of just painting stuff like a goon then i probably would have used this
