Detecting counterfeit guitars can be very difficult depenting on the quality of the copy. You need to read alot about the particular guitar you are considering. There is no substitute for doing your homework. Look at the "fit and finish". Knockoff are ofter of lesser quality. Removing the neck on a fender can show dates and such. Looking at the dates on the pots and matching this up with the date of the guitars manufacture (serial numbers) can help. Dealing with know reputable dealers is always recommended. One last thing, if the deal seems too good to be true, it likely is.

Some copies out there nowadays are so close, it can be difficult even for the well seasoned. Plus you gotta ask yourself, if the guitar plays well what does it really matter. Ok on resale if may, but the bottom line if it plays well and feels solid the by definition it's a good guitar