If it is an ARC entity, just use the LIST command.
If it's an arc segment of a polyline, you can use the "measure>distance". Specify the first point, then choose the option "M" (for multiple points), then choose the option "A" (for arc), then choose the option "S" (for Second point), then pick a point on the arc, then pick the other end of the arc.
Another way would be to use "measure>Angle" to get the delta, then use "measure>radius" to get the radius. The arc length can be calculated like this: L = R * D (in radians). So my example arc has a radius of 10.255, and a delta of 130° (which is 2.269 in radians)
10.255 * 2.269 = arc length of 23.2685
