Ny way to tell that a Care Package is booby-trapped

Using the Guardian effectively is quite difficult now that they aren't new to players any more. It's on par with the Sentry Gun from previous editions in terms of toughness. Your best bet is to deploy them around the corner from enemy spawn locations. Another good strategy is to have them backed into a corner so that nobody can get them from behind.

In terms of counteracting a Guardian, this is relatively simple. To find them you can either use the Engineer perk, which is particularly helpful as you can see them through walls and floors and will have a very obvious red shading and outline, great thing about this perk is that it also highlights Sentry Guns, Claymores and Bouncing Bettys. You can also hear them when you are close because they make a sound as if though they are emitting radiation, but this isn't very reliable amid the noise of combat. Perhaps in Search and Destroy this would be a good method.

In terms of destroying them, you can shoot them (on hardcore, this only takes a couple of clips), use a Black Hat tactical, which is a pretty nifty tool couple with the Engineer perk. It will hack the enemies equipment and destroy it (Sentry Gun or Guardian) or, with Claymores and Bouncing Bettys, make them yours. Lastly, you can use a Flashbang and temporarily stun it, and you can use this time to run behind it and safely shoot it or get past it.

You can press down or up on the D-Pad to cycle through your scorestreaks. Whichever one is highlighted in the scorestreak list is the one you will use next.

If you get an item via a care package, it will be in a fourth slot at the top of the list. Otherwise, they will be in the order they are unlocked.

Whichever one you've unlocked most recently is the one that will be used by default.