The only thing you have to do is to choose the folder where the fonts are. It doesn’t matter if they are installed or not, it will show you all of them. And you can write something to see how it will look like.
By this way, you will be able to see how it will look like and you will not need any third party program.
FontViewer will be very useful when you download a lot of fonts from internet and you want to see how many of them you are going to install or if you want to write something special and want to look for a certain font.