Unlike the earliest dovetail joints, which were irregular because they were cut by hand, the machine-cut dovetail joints were precisely created by machines for a refined look.
Look closely for irregular saw cuts or a rough appearance, indicating the desk was likely one of first roll-top desks and built prior to 1890.
You might also find the name of the manufacturer underneath the desk, which could even include the date the piece was manufactured.
Newer roll-top desks or pieces manufactured to mimic authentic antiques typically have no manufacturer or date markings.
It is common to see cuts and nicks with furniture where hand tools such as chisel were used.