[NTG-context] how to ask lua if glyph nr. x exists in current font

Hello, I'm trying to decrypt the LuaTeX reference manual. I find it easy enough to read data about different fonts when using fontA = fontloader.open('a.otf') and then glyphs = fontloader.to_table(fontA).glyphs but I'm not able to figure out how to aks the same question about the font that is currently being used (in MKIV). I would like to know if a glyph with some Unicode value is present in the font or not. Thanks for the hints, Mojca