Epinn,
Here's one way, not orthodox but here it goes.
Select your table, ungroup it, click yes on the warning message. Click
outside the table to deselect, click on the line you want to know the
thickness of, go to Format - Autoshape - lines and colors tab - you will
find the exact measurements of the line there. Then undo to regain your
table.
That said is it worth all the trouble and effort, maybe you would be better
off just reapplying the borders to the whole of the table in one fell swoop.
Select your style, tickness, color, click on the listarrow next to the
border button, click on second icon of the first row (all lines).