Every time there is a post about smurfs, it involves a story with a "smurf" wrecking a game. After reading the story, I decide to look up the game in question by using the details provided. From your case, the nidalee went 23-4-0. A hefty score for sure. HOWEVER, that doesn't mean the person is a smurf. In this instance, i would highly doubt that the person is a smurf. They were silver last season, they're silver this season, and they have almost 500 normal games played. My point is that doing well in a game doesn't equate to smurfing, just as doing poorly or feeding in a game doesn't equate to being boosted.
So with that, I want to point out a few ways to spot a smurf.
1. Generally, A smurf account will have a very low number of normal games played. This is because the account will most likely be used in ranked, and once the person hits level 30 they stop using it for normal matches.
2. A smurf will actually be pretty close in mmr to the main account. Unless the player is intentionally throwing some of his or her games in order to remain at a certain elo, it's easy to win games and climb quickly.
3. As a consequence of number 2, winrates will generally be high. However, high winrates doesn't necessarily mean that the person is smurfing. This account is my first account, with almost 4k or more games throughout all of the queues. I have above 50% winrate on my top 7 played champs in ranked, and most of them are above 60%. (http://na.op.gg/summoner/userName=Sax%20Man%2010).
4. Very few rune pages. You know how expensive they are. If they have a lot of pages, they're either spending a lot of money on the account (in which case they still wouldn't have a lot of pages filled), or it's not a smurf.
There are also a few common misconceptions about smurfing.
1. In most cases, a smurf will not blatantly announce being a smurf. If they are truly smurfing below their actual mmr, then they don't need specific roles to carry every game. The people that say "im an X smurf, give me mid i'll carry" are definitely NOT a smurf. Every single time someone has said this in champ select, I look up their profile. it's accompanied by thousands of normal games played and hundreds of ranked games each season, as well as terrible winrates with all of the "main" played champions.
2. Smurfing is not just to dominate lower level players. As someone mentioned in a previous reply, some people do it to try unfamiliar roles, some people get their mains banned, some to play with a friend. Now, this point isn't 100% a misconception; there are people that just want to pubstomp lower level people. At the same time, those people are the ones that will rage in game at the slightest mistake. When that happens, report them. They're likely to get banned. The reason i added this as a point is because it's not the main reason for deciding to smurf, at least not from actual upper elo players.
3. There are not smurfs in every single game. As i said earlier, winning a lane hard happens in EVERY ELO, even diamond and challenger. Also, in the leveling up process, not everyone does exclusively normal summoners rift games. a lot opt to spam bot games for a quicker win.
4. If someone is toxic on a smurf, it's pretty likely that they're toxic on their main too. If someone is respectful on their main, smurfing doesn't innately cause them to become toxic.
Now I'd like to talk about my personal smurfing experience.
I can't remember why I originally decided to start my smurf, but I do remember that I was somewhere in silver at the time. I played mostly bot games to level it up until around level 20, and then started playing normal games and doing twisted treeline. When I first started ranked, I used it to duo with my brother that was in bronze. At the time, my main was about gold 2, I think. I did not play my main champion pool or my main role while duoing with my brother, and I used that as a way to teach him how to learn and improve on his play. I went 7-3 in placements and placed silver 2, and continued duoing with him until he reached mid silver. I started playing on my own, and quickly jumped to gold 3, where I decided to stop playing on my smurf for the season and got my main to plat 5, where I decided to stop there, too. Throughout the whole time I was smurfing, I didn't rage in chat, and I didn't announce that I was a godsend and would carry everyone. If someone asked if I was a smurf after a game, I didn't lie. I tried my best to be courteous and helpful to my teammates, as I do on my main. This season I decided for the most part not to duo on my smurf and I placed into gold 3 (i think i went 8-2 or 7-3). I placed Gold 2 on my main. This refers back to a previous statement i made, saying that a smurf's elo should be pretty close to that of their main. In fact, I reached gold 1 on my smurf before my main, and now both accounts are at plat 5, with my smurf's elo/mmr being much higher than that of my main. Here is the link: (http://na.op.gg/summoner/userName=king+of+pangea)
tl;dr Smurfs aren't necessarily bad terrible people, and doing well in a few games or with a few champs doesn't mean the player is a smurf. Read the post, it'll only take a few minutes.