click the bookmarklet, then click the text whose font-family you wish to detect

detect-font-bookmarklet.js

javascript:onclick=function(t){return%20onclick=t.preventDefault(),alert(function(t){if(t){for(var%20e=document.createElement("span"),i=document.createElement("canvas"),n=i.getContext("2d"),l="",o=33;o<127;)l+=String.fromCharCode(o++);var%20a=getComputedStyle(t),c=[a.fontStyle,a.fontWeight,"16px/1"],h=a.fontFamily.split(/\s*,\s*/);e.style.font=c.concat(a.fontFamily).join("%20"),e.style.position="absolute",e.style.visibility="hidden",e.textContent=l,document.body.appendChild(e);var%20d,r,g=e.getBoundingClientRect();i.height=g.height,i.width=g.width,n.textAlign="left",n.textBaseline="top";for(var%20f,m=h.length;m--;)n.font=c.concat(h.slice(m).join(",")).join("%20"),n.fillStyle="white",n.fillRect(0,0,g.width,g.height),n.fillStyle="black",n.fillText(l,0,0),n.getImageData(0,0,g.width,g.height),(d=h.join.call(n.getImageData(0,0,g.width,g.height).data,""))!==r&&(f=h[m],r=d);return%20document.body.removeChild(e),f&&f.replace(/["']/g,"")}}(t.target))};