How do you tell if a book is an ARC book or not???

It will usually be printed on the front cover, back cover, spine (one or all places) saying "Advanced Readers Copy". Sometimes it will announce the publication date. It is usually quite obvious in my experience.

Thank youi Sally 

They often have publication data on the back. They are often paperbacks with the hardback ISBN. 

Book Clubs Versions are often slightly smaller books without the price. 

As Sally said, it is usually quite obvious - but not always.  I have 2 books in my possession right now that are ARCs that I didn't realize for quite some time.   One of them only has the words "Advance Readers Copy" in very small print across the top of the book on the back cover.  It doesn't jump out at you at all.  The other has a small print line across the bottom of the front cover and at first glance, I thought it was just a something like a review quote like you sometimes see on books until I actually focused on the words.  Just a caution that while most ARC's I've seen are pretty obvious with the words being large print and noticeable, I've run across a few that aren't.  I can totally see how someone could accidentally post one of these 2 books not realizing it was an ARC, especially if they picked them up at a thrift store like I did.

A lot of the times an ARC is a paperback but the ISBN will bring up a hardcover, so if the format doesn't match when you go to post it could be a signifier that your book is an ARC (or that the publisher recycles ISBNs, but still a good thing to notice).

Sometimes ARCs are also identified somewhere on the cover as "advance reading copy, advance review copy, advance reader's edition, advance copy, reader's edition, uncorrected proof, or review copy", and will also often state "not for sale".

In our FOL used bookstore where I volunteer, some ARCs that get donated don't have any cover art and instead in big font it says 'ARC copy' so that makes it very easy to identify them!!!

It will be on the front, the back, or inside first pages.

as some have said, not always obvious.  I have one that looks like the normal cozy paperback that would be issued and it was in tiny tiny print that I saw the ARC wording.  Because I'm a book review blogger I sometimes get the books ahead of time and they are the books you will get off the shelf, but on rare occasions the print copy might be an ARC.  Heck one time i got a hard back book and the author wrote inside in pink highlighter that it was an ARC...umm, what author makes an ARC a hardback?!