What do I have to do to type the ≤ sign with Inkscapes text-tool? What tool or key-combinations would I have to use?

Note: I take it there are smoother ways of accomplishing it that go beyond copy & past. I'm in Ubuntu Linux, using Gnome.

I'm guessing that your best option is probably to use the Character Map. In Windows, it's in the Accessories file. With the Character Map, you can either copy/paste the symbol, or select it, and look in the info area at the bottom for the keystroke combination. In my initial investigation, I find that symbol is not available in every font.

Although please note that I have NO idea whether Inkscape relies on the Character Map for this type of thing, so it may not work at all.