Is an ARC always marked as such?

Yes Darcy, they will have some marking on them somewhere indicating they are an ARC. It will say Advance Reader Copy, Uncorrected Proof, or something to that effect somewhere on the book. It isn't always big or really noticable, sometimes just a little printing across the spine or cover somewhere but it will be there if it is an advance copy.

OK, thanks!

The quickest way to tell is to look at the spine - the publication date (month and year) is always listed on the spine.  There will usually be a note somewhere on the front cover, and there will be no ISBN on the back.