How can you tell if a 1950 US 50 dollar bill is fake?

US bills printed before 1990 had very few anti-counterfeiting features so it's not as easy to spot fakes. For example, there's no security strip, watermark, or microprinting. Instead you can look for these qualities:

• The paper should have a solid feel to it, similar to the paper in modern bills.
• If you hold the bill up to a light there should be tiny red and blue fibers embedded in the paper.
• The portrait of President Grant should have uniform engraving marks and exhibit an almost 3D appearance when viewed from a slight distance.
• The "dentils" (teeth) around the Federal Reserve seal should be sharp and evenly spaced.

That said, if your bill is very worn it doesn't have a lot of extra value unless there's a small "E" next to the date; you can find more specific information at the related question. If the bill is in better shape you might be able to sell it to a dealer or on eBay.

Remember that banks are NOT permitted to pay a premium for old or rare money; a common mistake is to take an old bill to a bank hoping to get more than face value. In your case all you'd get is a nice, new $50 bill in return.