Take a good, close look at the hardware holding the back boards in place and the hanger hardware. If it is 18th century, these should be very clearly hand forged, and the nails should be a square, homemade hand-cut type, all of them just a different from each other.
This doesn't mean it's 18th century, it could be 19th, but you can tell a whole lot from the hardware and know if its old enough that folks were making their hardware at home, or if they were buying mass-produced manufactured hardware as they did in later years.
Also keep in mind that you may see a screw or two that was added at a later date than the original hardware.
I have some hand-forged hardware on the back of an old federal mirror that has been in my family for generations. I'll post some photos for ya. Back in a flash!