Test if a Font is installed

string fontName = "Consolas";
float fontSize = 12;

using (Font fontTester = new Font( 
       fontName, 
       fontSize, 
       FontStyle.Regular, 
       GraphicsUnit.Pixel)) 
{
    if (fontTester.Name == fontName)
    {
        // Font exists
    }
    else
    {
        // Font doesn't exist
    }
}

How do you get a list of all the installed fonts?

var fontsCollection = new InstalledFontCollection();
foreach (var fontFamily in fontsCollection.Families)
{
    if (fontFamily.Name == fontName) {...} \\ check if font is installed
}

See InstalledFontCollection class for details.

MSDN:
Enumerating Installed Fonts