When I see a site that was designed using the many templates out there (such as templatemonster.com) it is very easy to spot. What do these templates have in common that makes them easy to seperate from custom designed websites?
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I can give you an opinion from the perspective of a graphic designer.
The fact that it is a template suggests that there are only a limited number of designs, with perhaps a choice of colors. A person, seeing these designs repeated many places will recognize the patterns and know them to be from templates. An easy example will be the choices offered for MySpace members.
The other thing about templates is that they are not designed for any specific use. For example, if someone pulled a template from a web design application for the purpose of featuring custom motorcycles, the design will not likely show anything reflecting the product’s use, market, engineering or anything about the maker.
A template designer almost have to create bland features to appeal to as many users as possible.
There is no way a person can automatically tell a template design merely by its looks. Good design is good design. High impact, effective design will have as meaningful a message as the content.