Does investigating cities show locations of resources within the city limits, even when you can't ordinarily detect the resource? This is interesting. I would never have suspected this.
As Elephantium said, you can see the extra shields that a resouce gives, even when you don't have the tech required to make the resource visible.
You cannot tell, if there is oil when you do not have the tech. As was mentioned, you may infer it via shields and gold. The problem is that the three tiles types are not the only consideration. It is possible that they could have cleared a marsh and still get the oil.
But does the AI clear marsh ever? Even if it does, I believe it is only after the AI workers have finished any other job.
The time frame puts a squeeze on you as you must go fairly soon......
This is very true and some nice tips from vmxa.
Another reason to start raizing cities as soon as possible, is that it will probably put Portugal in mobilization mode.
Even if they do not mobilize, you still have the time to win by Domination (start from their island and rush culture to claim all the coastal tiles in your continent).
Also, you could use a trick to get rid of many of Portugal's bombers.
If you gift them a city near their continent, the AI probably will move in their bombers as long as they don't have a target at home.
So, you could immediately sign peace treaty with gpt to Portugal for their cash and Corporation (could even try to get Ironclads, even if they are too slow), gift them Leiden and then make them to declare. Wait 2-3 turns and retake the city.
Probably that will also make Portugal to leave Amsterdam and start landing at Leiden (flat ground). Just be sure to protect the cities surrounding Leiden with barricades (stop any tanks for those turns before recapturing Leiden).