I am a big Gucci fan and lucky to own some bags. Here's what I can tell you about looking for a genuine Gucci bag and purse:
There is always a leather tag under the inside zipped pocket. The front of the tag reads GUCCI Made in Italy and on the reverse will have two line of 6 numbers. All this is stamped into the leather tag. There is also an R in a circle.
Check the inside zipped pocket. In one of the corners you will find a black label with GUCCI and a serial number on it. This tag does not come with the small messenger bag where the inside pocket is not zipped.
The zip pulls are leather and always the same colour leather of the bag.
Care card and contollato card
Both cards are white:
The controllato card reads GUCCI and underneath controllato are the number 1 2 3 4 5 6 7 8 9 0.
The care card is folded into four with instructions on how to care for your bag.
If you are buying a new bag from Ebay as new make sure these are included. If it is a second hand bag you are buying these may have been lost.
I know of three versions of dust covers that come with the bags:
Dark brown satin feel with the GG logo all over it.
Plain golden brown coloured with GUCCI written in gold. The inside label reads GUCCI and 100% cotone - cotton. The drawstring ties on the top right hand corner.
Plain dark brown with GUCCI written in gold (the newest version). The inside label reads GUCCI and 100% cotone - cotton. The drawstring ties on the top right hand corner.
Other tips and things to check for:
Gucci bags never have plastic wrapping around the bag or the handles. If you see a bag with this then it will be a fake.
Check the stitching and the quality of the lining. If the lining is flimsy then it will be a fake.
If you find a bag with any other dust bags than those described above it is probably a fake.
New bags don't tend to have a Gucci card tag attached as they are normally cut off in the shop when purchased.
To main things to check for:
Snap closure: I have two purses and both have GUCCI written on the snapper.
Serial number: There is always a serial number normally located under the credit card slots. There are 6 numbers followed by a dot followed by 4 numbers ie 143389.4276.
Hope this has been of some help.