Click on a province. Look at the new UI window that pops up at the bottom of the screen. Next to the name of the province, there should be a bunch of faces with numbers next to them; hover your cursor over them to see the total population of the province (it should say something along the lines of "There are X citizens/freedmen/slaves (XY% of Y people)" or something like that). There needs to be 10 or more pop all up.
For civilization value, on the same UI screen there should be a percentage value just under the number of people in the garrison.
Besides if more than one country fullfills the previous criteria only the country with the most civilized province next to the provinze to colonize can colonize it
Also countries who pay tributes can't colonize any provinces whatsoever but nonetheless if their civilization is high enough they can prevent colonization by other countries to provinces next to them.
Note that this only applies if you're using the Vae Victis expansion. If you're still playing Rome 1.3, it's just the country with the quickest trigger finger who gets to colonise that province first.
Also note that if you are playing VV2.1, your governor's Finesse (or was it Charisma? Can't remember) characteristic is added to the province's civilization value when determining who gets preference for colonising a province. Just another thing to keep in mind if neighbours keep butting in on your potential colonies.