I have always wondered who a rapper is that is featured in a song. For example, when the rap version of Bad Blood came out...I had no idea how to identify the rapper. I couldn't find anywhere that could help me identify him. What are some techniques I could use to identify rappers on the spot?
If all you have is the audio it's all about experience and recognizing. You have to know the rapper's voice and be able to differ it from other rappers. With videos it's a bit easier, because you may have seen the rapper in a movie or in the news before and recognize his/her face.
If you don't listen to a lot of rap you probably won't be able to recognize the voice. In this case you first want to find out the title of the song. Car radios usually support RDS and depending on the radio station, the song title will be displayed on the radio screen. Some radio stations also display features and artist names, so you could find out that way.
If you already know the song's title, thanks to the internet, it's easy! Try simply googling the artist's name and song. For most songs there's a Wikipedia article which will tell you exactly who wrote the lyrics, who produced the song and whose voices can be heard.
Another approach would be a site for lyrics. I can recommend Rap Genius for this. It shows you the producers and writers as well as who delivers what part of the song. It also gives you a lot of background info via annotations. However, it is all based on the community. So if you don't find a song on Genius it's either not that popular or no-one has put effort in providing the internet with lyrics.