Not every location is suitable for settlement.  You'll get the most benefit by casting it in swamps or those dead-looking forests, I think.  But yeah to answer your question, if you don't see any grain/material/essence yield info after casting, it's not a good spot.

Best used on brown terrain (plains) that's an appropriate distance from any city (can't built too close to another city). Those usually provide a spot to settle. Even better if you do it on or near a special resource (bone ogre lair often yield 3 essence tiles for example).

It doesn't work on: desert, city (razed cities) tiles. You also need a min of 6 workable tiles to build a city, so don't cast it against a cliff or in a corner.

You can easily tested this spell out by taking cheat mode by first Ctrl+U to turn off the fog, then Ctrl+R to unlock the spell, and finally press Ctrl+M a couple of dozen times so that you have enough mana to cast anywhere you like.

Some of my exp:

1. Like Schweiz mentioned above, it has hard tile restriction to nearest city, 7 tiles on huge map, 6 tiles on medium/small, not sure about tiny.

2. Your city buildings also has a restriction to the nearest city buildings, 4 on medium/small (aka, at least 3 spacing in between). Otherwise, you may have a city that you can't build any buildings.

3. swarms, seas, oceans will never turn to buildable tiles, don't waste your mana on those places.

4. tiles close to a resource has best chances, even deserts, note, not swarms, though.

5. tiles close to river might has some chances, try it out if you don't have a choice.

6. a good city snaking can do the same. city influenced area is auto re-birthed. why waste mana?