Sometimes it's uncertain whether a card is an attack or not. The official cards can be explained; Masquerade isn't an attack because of rules confusion with Moat, and Tribute and Possession only affect the player to your left and not in a negative way. IGG can't be an attack as an on gain effect without some confusing "gaining this card is akin to playing an attack" wording. The compulsory non attack interactions are generally seen as positive (eg draw a card, gain a silver) and the mixed ones like Bishop and Vault are optional.
However I have issues with a few of my cards that don't have obvious answers:
Annex
Action/(Attack?) - $4
Each other player may discard any number of cards from his hand
Draw until you have twice as many cards in hand as any other player
Now this card is obviously nasty, but you don't technically have to do anything, so you aren't necessarily being attacked. On the other hand the nature of this card makes it seem appropriate to be able to use reactions like Secret Chamber and Beggar.
Pariah
Action/(Attack?) - $5
Gain a card costing up to $6. Each other player may gain a curse, putting it on top of his deck. If he did, he also gains a copy of the card you gained.
It's all in your opponents hands, and both options (nothing vs curse and $6 card) are meant to be viable and better in some situations and worse in others. One of them, however, if you take away the "wash" effect of gaining, is most definitely an attack however. Similarly, would soothsayer be an attack if Curse+Card was optional?
Conqueror
Action/Attack - $5
Gain 2 cards with a total cost of up to $7. Each player (including you) looks through his discard pile and either trashes a card from it or puts a card from it on top of his deck.
This card is almost always beneficial to your opponent, but if, for example, their supply pile entirely consisted of Victory Cards (that they want), then it's an attack. It's probably an attack as often as Governor's Silver. This ambiguity can of course be solved by making the effect optional (and you get the 3 letters back by getting rid of the "es" in trashes and "s" in puts).
Prospector
Action/(Attack?) - $2
Gain a Treasure costing up to $6, putting it in your hand. Each other player gains a copy of that treasure, putting it on top of his deck.
Now this card, if you choose to gain Copper, is most definitely an attack that hurts. However if you reveal a moat, then your opponent gets to choose to Gain a Gold in hand without benefiting you, making you regret revealing the reaction, so the optimal play when you have a moat in hand is probably not to reveal it. Similar effect with Lighthouse. Almost makes it worth not being an attack. Also of all the treasures you can choose, only one of them would almost always be considered an attack; triggering Beggar then being given a Gold seems like a strange interaction indeed.
Please let me know whether you think each card should or shouldn't be an attack, and I'm interested in hearing edge cases of your own.