It is very funny but I can spot an American from a mile away. So I thought I would tell you why/how (plus someone asked me to). I asked my followers to help me with their observations. I also have to be very polite here because it isn't always a nice way of putting the differences.
Baggy jeans (American styles see to be baggier even though they are not as baggy as they used to be). American men tend to wear baggy cargo shorts or basketball shorts with logos.
Light coloured jeans
Americans tend to wear t-shirts. British women wear blouses often with a cardigan. The t-shirts that the Americans wear tend to have large logos on them. British men tend to wear more collared shirts and t-shirts with small logos on them.
British seem to reserve running shoes for running. More likely to wear Converse, Vans or Keds as everyday shoes.
Baseball hats- it was noted that when a woman wore a baseball hat, she got comments because it is not common.
Sports team jerseys
Americans are a bit more casual- yoga pants. Although it was noted that the younger British seemed to wear track suits .
British Women wear casual dresses as everyday wear.
If invited over for dinner, dress to impress.
The women wear scarves, so get one or two to add your wardrobe.
During my research I found this article which was helpful and very true!