How do you detect a counterfeit 1934 A US 20 dollar bill?

US bills printed prior to the 1990 redesign had very few anti-counterfeiting features. Among other items they didn't have watermarks, security strips, color-shifting ink, or microprinting. About all you can look for would be:

• There should be small red and blue fibers embedded in the paper.
• The "teeth" around the bill's seal(s) should be sharp and evenly-spaced.
• The engraving should be crisp and clear. In particular the portrait should have an almost 3-dimensional appearance.