How do you know if a 1977 us 100 dollar bill is real?

Older US bills have essentially no high-tech anticounterfeiting features. Things like watermarks, security strips, and so on weren't introduced until the 1990s. About the best you can do at home would be the following:

> Look for tiny blue and red fibers embedded in the paper. These are tiny bits of very fine thread that were mixed in with the paper because they're difficult for a counterfeiter to reproduce.

> Check the teeth (points) around the Treasury Seal. They should be sharp and evenly spaced.

> Look closely at the engraving of Ben Franklin's face. Again, the lines should be sharp and clear. The image should have a certain amount of apparent depth to it.