As mentioned the dark vengeance marines are the only ones with sculpted shoulder pads and therefore specific to Dark Angels. All other standard marines are just ultramarines for the studio scheme. if you are going specifically DA then it may be worth looking up the codex to find the units. If it is just to know which models the faction uses, then just look under dark angels on the 40k section of the GW website and that will list everything they take. You can buy specific conversion packs that come with nice chapter specific heads and other bits.
If there is a normal marines unit that dark angels don't get access to, such as some of the vehicles, there is nothing stopping you allying in a standard marines detachment which are counts as Dark Angels. As all imperium armies are battle brothers it won't have much affect on your army construction, though you would need to get the adeptus astartes codex as well as dark angels to do that.
Me: To be honest im amazed there isn't a chaos god of not revising or at least chaos god of procrastination
MidnightSun: There will be, when enough people do it. Y'know when the Eldar were all engaging in their hedonism and Slaanesh ripped open the Eye of Terror with his creation? Slaanesh will have NOTHING on the God of Procrastination, and his vile minions will carve the galaxy asunder in a thousand year reign of blood.... Tomorrow