Here are some ways you may be able to check to determine if your note is from the colonial era.
I should mention recent reproductions include red ink but have the word facsimile in very small print along a margin border. Older reproductions (from the 1960's or 70's) will not have any statement that they are reproductions but can usually be detected by their artificially yellowed appearance. An example of a $20 reproduction has been enclosed as an attachment.
Beyond these steps you should take the notes to a dealer for verification.
The standard guide is Eric Newman, The Early Paper Money of America, 4th ed. Krause Publications, 1997. In that work the retail values of the $8 note from the May 10, 1775 emission are as follows: Good $20; Very Good $30; Fine $45; Very Fine $65; Extremely Fine $90; Uncirculated $180.
If your issues are Continental Currency or from Pennsylvania, genuine notes from these issues were made from a special paper which contained thin blue threads as well as mica flakes. Continental Currency from the emission of Jan. 14, 1779 have the watermark UNITED STATES in the paper and can be seen if the paper is held up to a light.
Many reproductions exist. In fact packets of 5 different notes are sold for $1.00 at Williamsburg and other colonial sites.