RGT said:

the symbol above an open 'tile' (resource location) will be grey - if someone else is in the tile it will be red, and if you click on it the other player will show. If you send troops to a tile that someone else is already in your troops attack theirs - if your kingdom has a non-agression agreement that will make the other perosn angry because he or she thought their troops were safe while yielding

Check their order code too, though.

Some orders have a 'right of first march' and others a 'right of first arrival'.

For the former, if you see tracks before you begin your own march ...

(And get there first ...) [Problem is ... proving who marched first, after the fact.]

With 18 marches, myself, I certainly am not doing to check tracks first. At least, if shielded, you'll bounce. Which gets old ... REALLY fast ...

Lord Cranstoun said:

How can you find out where someone is farming? Apparently I pissed someone off and their picking on my farming attempts.

The real answer is, you can't. Not without actively looking everywhere and checking each occupied tile.

There are a few things you can, and many, do ...

• Leave your map over the enemy's town. When they march, you'll see the tracks. Click on the march and the map will scroll to keep the march centered. Upon arrival, your map will be centered on their tile. (Problematic when many marches are happening simultaneously - you can only follow one at a time this way.)
  (This is one reason why march speedups and boosts get used - once the march arrives the tracks disappear. If the tracks weren't noticed, your yielding is essentially invisible.)

• The watchman has a radius of 60m / diameter of 120m. It is reasonably trivial to check all the tile types with it looking for your town and order. Once found, attacked. [Since tiles only need 1 informer to be espionaged, it is easy for them to detect which troop type is there, and how many, and send accordingly. In essence, short of safe conducts, TILES ARE INDEFENSIBLE! Even if you send near 2m troops, they can do the same, with boosts even, with the troop type you are most vulnerable to.] 
  - since this diameter is large enough to cover all alpha, 6, and 5 areas, the tiles that completely yield in a less than completely stupid time, an order mate can sit shielded near the ES, and call coords for others. [Attacking an occupied tile drops shield.]
  - this is a common practice during OvO and KvK.
  - yield speed is why alpha camps are attractive, 6 less so, and 5 rather less so. 4 and under are right out - yielding takes too long and so few troops are needed to completely yield the tile it makes the cost of dropping shield to kill so few uneconomical. This means that you really only need to check the watchman for these higher levels, reducing the time needed to check. 120m is plenty to cover these higher levels.

• Port next to the miscreant, and use the watchman as above.
  - defended against by only yielding greater than 60m away. ('course, you can't then use your own watchman to find desirable tiles in the first place ...)