Unless you've managed to get a switch made out of superconducting material and you're working inside a liquid helium bath, everything has resistance :)
You'll have a bad time trying to measure such low resistances, so usually you can look at the datasheet for the switch you're using or for a similar switch if you need a ballpark number. You'll see an on-resistance, and a max current (since there will be arcing when you open the switch).
If you do want to measure the low resistance, you'll need dedicated meter with a kelvin connection.