Early Indication your Nieghbor is going to attack

[Mutineer]

1) they are friendly.
2) they ask for embassy.
3) they ask for open borders.
4) they refuse to make declaration of friendship dispise listed as friendly.

Build you army they are coming.

 

[phillipwyllie]

Aside from the obvious troop shuffling on your borders, and/or the civ involved you can't really tell for sure.
In my last game I spawned near Maria. To prevent her from forward settling me I DOWed here early to take part in some worker stealing. Once all that was done we had a perpetual DOF, which i had with most civs. Fast forward to the modern era and I'm 8 turns from Plastics, and having unlocked gunboat diplomacy I was planning on building as many infantry as I could support and invade Portugal. I decide to denounce early after the DOF had ran out but when I clicked next turn both Maria and Theodora DOWed me, needless to say I lost Honolulu(and recaptured).

 

[NASAROG]

Yep King of Nukes
Better watch out when he's in your game.

1) they are friendly.
2) they ask for embassy.
3) they ask for open borders.
4) they refuse to make declaration of friendship dispise listed as friendly.

Build you army they are coming.

You must have been playing against India, eh?

 

[Mutineer]

You must have been playing against India, eh?

Just watch for that, any civ does that. I am not joking, it is result of long time observation.