Every single one of the cards in the fake set is quite darker than the original set, and every single one of the cards in the fake set has also had some of the photo details trimmed off on the left/right sides when compared to the originals. On a few of the cards, the images have been enlarged to fit the cards, but on a few some photoshopping was done and one has quite a comical affect. All of the cards have differences from the originals, but I am going to focus on 5 "easy" tells to help you out.
Lobby Card #7 - Crew Portrait
I start with this one, as it is one of the easiest ways to spot a fake. Basically if Harry Dean Stanton has an extra right arm floating next to him, you have a fake card. (there are also some photoshop effects on the right side). The last few centimeters of the image on the left side of the card were duplicated to extend the image to the borders, and in doing so they made a duplicate image of part of Harry Dean Stanton's arm/jacket (and then darkened it so it is harder to tell).
Card #7 Real 
Card #7 Fake 
Card #7 Side-by-SideLobby Card #8 - in the Ship
The easy tell on this card is the light board behind Sigourney Weaver's head. If you have a card where the light extends all the way to the border, you have a fake. On the original the light ends just before the border, while on the fake the edge has once again been photoshopped to extend the cropped image to the border.
Card #8 Real
Card #8 Fake
Card #8 Side-by-SideCard #1 - on the planet surface
This card is just a cropped image, so there is missing artwork on the left and right borders. On the right side not as much as the alien machinery can be seen, while on the left the easiest tell is the small white triangular piece coming into the middle of the card on the arm of the spacesuit. On the original it is much longer than on the fake.
Card #1 Real
Card #1 Fake
Card #1 Side-by-SideCard #4 - Alien Gunner
Once again slightly trimmed art and some photoshop effects. When looking at the alien wall design on the right edge of the card, on the original not only is there slightly more image, but the edge design tends to slant slightly up at an angle, while on the fake cards it just extends straight for the last few centimeters.
Card #4 Real
Card #4 Fake
Card #4 Side-by-Side (right edge)Card #5 - Walking thee planet surface
Another case of trimmed artwork that is photoshopped to bring it to the edge. The mountain on the right edge of the card behind Tom Skerritt should extend at an upward slope to the edge of the card. One the fake card the last few centimeters of the image have been extended along the right edge, causing an unnatural appearing pattern on the edge (right after the spacesuit ends) and causing the mountain behind him to sort of flatten out at the edge. Also the overall darkness of the fake card leaves out a lot of detail of the spacesuits that is barely visible on the real card.
Card #5 Real
Card #5 Fake
Card #5 Side-by-Side (right edge)Hope this guide is a little helpful to anyone looking to add this set to their collection, or anyone wanting to determine if the cards they currently own are original or not.
