Thanks for the question. There are really only two possible ways of telling if a wheel is forged or cast with 'naked eye inspection'.
The first which is normally the biggest tell of any cast product is the parting line. Every cast product is made with a mold that consists of two or more pieces. In EVERY cast product, the place where the two molds come together is called the parting line.
Once the piece is finished, there will always be a very small seam where the parting line was. Think of your childhood toys and how they always had this little thin line all around. Think of a toy rubber ball that has that small line that goes around it. That's the parting line. That is the biggest tell.
Sometimes, companies will go to great lengths to disguise the parting line. In a wheel, the parting line would not be towards the center of the barrel but closer to the face.
The only other way of knowing is simply by picking the wheel up. A 19" 9x5 forged Aluminum wheel should weigh no more than 20-23 pounds AT THE VERY MOST.
There are, however, manufacturing methods that combine casting and forging to form a hybrid wheel and produce very light pieces.
At DTMrack, we believe this technology will position itself over full forged due to the fact that wheels can be made just as light as most full forged wheels, without the SUPER excessive cost to the customer.
If there are any other questions you might have, please do not hesitate. We are here to answer as much as we can so you can make the best decision when shopping around.