is there a "dimension" tool -- not apparent in manual or on toolbar

Please keep in mind that Inkscape isn't a CAD application [1] - its support for technical drawings is still limited (but improving with each release [2]). Quoting from an earlier answer to a similar question about dimensions as known from CAD applications:

<quote>
You can create dimensioning lines in a two-step process, but - unlike in most CAD programs - the dimensions are not updated dynamically when the geometry or position of the measured objects is changed:

1) draw dimensioning lines with 'Extensions > Visualize Path… > Dimensions…'
<http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Extensions-GenerateFromPath.html#Extensions-Dimensions>

2) add lengths to the dimensions: Select each dimensioning line you want to display the length for (use 'Ctrl+left mouse button' because the lines generated in step 1 are inside groups), and use 'Extensions > Visualize Paths… > Measure Path…'. The extension will add the length as string 'put on path' to the dimension line.
<http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Extensions-VisualizePath.html#Extensions-MeasurePath>
</quote>
<https://answers.launchpad.net/inkscape/+question/128630>

Some additional tips for measuring paths/objects with the current stable release (0.48.1):
<https://answers.launchpad.net/inkscape/+question/142600>

[1] If your drawings require high precision, Inkscape might not be the best choice (internally, all units are stored as SVG user units (px) in the SVG source. The conversion between these SVG user units and the chosen document default unit can introduce rounding errors. The length of the SVG user unit is not absolute - the same SVG file might be scaled differently in different SVG viewers or applications able to import SVG files).
Inkscape also lacks the feature to set a drawing scale: like when drawing on paper, you have to calculate the dimensions based on the intended drawing scale yourself.
Other often requested but not yet implemented features known from CAD applications are drawing aids like Trim/Extend and Fillet/Chamfer tools.

[2] There are already many features implemented which are very useful for technical drawings: multiple configurable grids, various snapping modes and targets, guides, constraints when moving/transforming objects or nodes, alignment & distribute options, line types (dashes) and markers, layers, etc.

Sorry, the quoted answer uses an old, no longer existing link. Here's the updated link to the manual section about using the extension to draw dimensioning lines:

1) draw dimensioning lines with 'Extensions > Visualize Path… > Dimensions…'
<http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Extensions-VisualizePath.html#Extensions-Dimensions>