Hi
I am just as fed up as everyone else with the fake A&F on this site. I have my own ways of spotting fakes, which begins with the label, which should be rectangular and white or cream and sewn right into the garment.

One surefire way to check whether you are looking at a genuine A & F item is to compare with the items on the official Abercrombie and Fitch website. There is a UK version for garments available in the UK and don't forget that these are exactly the same as the ones available in the USA. However not all items are available here as some stores in the US do sell items notfor sale here.
Occasionally some US stores do have special one-offs, but these too will have the right labels in and not the triangle ones.
Women's t shirts tend to be fitted, so they will not be "t" shaped or boxy shaped, as many of the giveaway fakes on ebay. also, all A & F items have names  The seller of a used top they have bought themselves will know the name of the garment. If they are selling a BNWT garment and cannot tell you the name then it is NOT genuine.

A & F garments generally have embroidered (appliqueed) logos, in addition they will have taped seams inside. Many of the garishly coloured  tops, starting at 99p will be fake. No-one is going to start an auction for a BNWT top they have paid £25 to £30 for at 99p!   
Look for the items which have a name, and these will be the genuine ones.

Ask questions and ask for additional photos, particularly when buying Buy It Now items which are expensive.

If in doubt do not buy.