If the Date cannot be determined but you can determine that the coin is a "Small Letter" Variety (wide spacing between the "A" and "M" in AMERICA), then it can only be an 1858 "Small Letter" Flying Eagle Cent! Dies with this spacing between the "A" and "M" in AMERICA were only used in 1858 but Dies of the Large Letter Variety (little or no spacing between the "A" and "M" in AMERICA ) were used both in 1857 and 1858. There were no Dies of the "Small Letter" Variety used in 1857. I hope this is not as confusing as it reads to me!
In other words, a Flying Eagle Cent with little or no spacing between the "A" and "M" in AMERICA (Large Letter) and with no discernible Date, can be either an 1857 or 1858 Flying Eagle Cent but if there is wide spacing between the "A" and "M" in AMERICA (Small Letter), it can only be an 1858 Flying Eagle Cent.
Frank