System > Preferences > Appearance
Click on the Fonts tab

oh yes - there it is,
the default font was Sans 10
any suggestions for one which discriminates between 1 I l etc?

If you don't already have it installed, I'd say install "Specimen" a font
previewer.

holmwood.id.au/.../

It should be available in the repository Synaptic - it lets
you type out any character you'd like to see within your chosen preview font and
font size.

Liberation Serif seems the best so far,
a lot of them have no noticeable difference between
numeral one and small 'L' and capital 'i'