CAD tip # 4780:

You can pick and sum up the individual vertices through the DIST command; you can use the AREA command (gives the full/closed length); the LENGTHEN command; or open the Properties window (Ctrl+1) to display the length (and optionally copy/paste it to the drawing, Excel or other application).

To sum-up lenghts of many lines (per-layer table) you can use the "Report Length" function from the LT Extension addon utility.