Tap any font file with .ttf or .otf extension in your file explorer to preview it.
You can also launch the Font Viewer app directly and browse to open a font file.
Font Viewer includes a slider to change font size and allow zooming on font details.

Written and compiled on an Android phone thanks to RFO BASIC! the GW library and the BASIC! Compiler:
RFO BASIC! https://play.google.com/store/apps/details?id=com.rfo.basic
GW lib http://mougino.free.fr/gw.html
BASIC! Compiler http://mougino.free.fr/compiler.html

Open source: http://rfobasic.freeforums.org/apk-extension-registration-t4587-10.html