I want to check if an item is in player's inventory(player has taken it before).
How can I do this?

Type and enter "i", "inv", or "inventory", or check the "Inventory" pane on the right-hand side.

If it's IN the player's inventory, then the object's inbuilt .parent stat will be Player and you can check for that. For example:

PackOfCigars.parent=Player

As for if it has ever at one point been in the player's inventory, you need custom attributes for that. Like, say, for PackOfCigars have 'BeenPickedUp' as a true/false. And when you pick up the PackOfCigars, under the 'after this item is taken' run:

PackOfCigars.BeenPickedUp=True

Hope this helps.

There's a core function Got.

Example:

if (Got (screwdriver)) {
  msg ("You unscrew the panel")
}
else {
  msg ("You need a screwdriver to do that")
}

You could also do Contains (game.pov, hammer) (this checks if the object is in the player's inventory, even if it's invisible or inside a locked container. game.pov is the current player object) or check if the item is in the list given by the expression ScopeInventory() (which is what Got does)