In my experience there is a range between killstealing, killassisting, and killsecuring. For example, I tend to play ADC, and don't mind giving a kill or two to my support when we're in lane (typically about 1/5th is the best ratio in my experience). I lose 150 gold and provide the support with an extra 150 meaning that they can do their job better and not risk becoming a feed for the enemy ADC if I get smoked by happenstance. (Meaning that, as a player, my kill streak may be less, therefore not awarding the opposing team a shut down bonus, along with scaling more beneficently with my support).
However there are exceptions to this as well; I don't mind our jungler popping in and landing a hit to grab some gold from the assist, but whats annoying is when they pop in, take a kill from the lane when it already was secured, and then leave when the opposing jungler appears. I cannot tell you how many times I've seen Warwicks use ults to suppress a target when there at 1/8th health and my Caitlyns ace in the hole is locked on. (They should have just tossed a light hit at them to grab some minor gold) rather then those shenanigans. These types of behaviors mark a kill steal.
Similarly, about level six, Darrius (and Garren via proxy) becomes a huge problem for this if you have the displeasure of being laned with him. I met a couple who take the assist using the ult to break high armor... but this is an exception not the rule. typically then after forcing his lane mate to become weak wonders why his team is losing. To Darius players, ult cool down ain't that bad. 100 seconds is a minute and 40 seconds. That means you're going to have it in EVERY TEAM FIGHT ALREADY, so use it to secure kills your team mate would have missed, OR use it to open fights. If you don't think like this, you become more of a gold pool for the enemy team rather then a help too you own. If you're curious what kill stealing looks like, just lane with a bad Darrius and it becomes apparent.
The best way though, ultimately, to find out who kill steals and who does not, is conveniently found on the STATISTICS page at the end of a match. Sure a first place kills with 17/5/2 looks good at first glance, but when you see that the player who got this is 3rd to 5th in total damage dealt to champions you can be completely sure they spent most of the game kill stealing rather then legitimately assisting the team. Therefore they took gold away from useful team mates. It's best to realize that the person with the most damage, most likely brought the team the most gold via tower kills, assists, ect., not necessarily the person who had the most kills. I've had matches where my teammates didn't understand that taking a tower nets the team 5x150 gold (that's 750 gold, more then 2 champion kills unassisted). Of course, there are exceptions to this. 0/15/2 should still make you wary even if they did do the most damage.
There's a lot to say on this topic, but it very much boils down to this "you know when you see it".