Fallout New Vegas features some items that are more or less obviously useless, like pots or ashtrays or burnt books or cartons or cigarettes or conductors... wait, conductors? That doesn't sound like junk.
Most of this stuff is in the Misc section of my inventory; some of them have a specific icon, others seem to have a generic "junk" image, almost all of them are in the Misc. section of the inventory.
How can I tell junk from stuff I can actually use in crafting?
Misc items are not totally devoid of use. There are certain quests which require misc "junk" items, including Conductors.
Various recipes also require these items. Notably, Tin Cans have use in a few quests (for one you need 100 pieces of junk metal... that's a lot of cans!) and in certain recipes.
Having a lot of empty bottles and Old World Blues can help you get Desert Survivalist. In fact, a lot of junk items have a lot of utility once you've fully upgraded the Sink in Old World Blues. (Potential spoilers if you haven't been to the OWB DLC yet, but see the Sink's Book Chute as an example)
Really, though, there are a lot of these misc items laying around, so selling a few of them isn't going to cause you a large issue. If you systematically collect all the things and sell them, you're going to end up in trouble, though.
In the long haul, the bigger problems are your carrying capacity and the amount of caps the merchants have. Most of these items are heavy and mostly worthless. Even if you only carry the light, valuable, useless vendor trash, you're probably going to have more value than you can reasonably sell, even at low Barter levels.
A lot of Misc. items are used in crafting. The junk icon hardly indicates anything here, sadly.
Primarily, I would recommend that you stick to wiki pages such as the one I linked, and keep track of the items you plan to craft (at the respective crafting tables etc.) - so you know what not to sell.
The Crafting Shopping List at the end of the linked article is also a great resource to look up - and to keep at hand.
It's also probably easier to remember what isn't on this list, than what is. Such a list can be extracted from here - Anything that doesn't have a purpose (such as being related to a quest, or being a component in crafting) can be freely sold.
And, quite frankly, in the early game you don't need to hoard a lot of items either. When you are finally able to craft them, you'll most likely have easy access - or remember easy access - to a lot of them anyway. Just don't pick up everything unless you know you need it. ;)
