Englishfathom
verb
1 Charlie tried to fathom the expression on his friend's face:
understand, comprehend, work out, fathom out, make sense of, grasp, catch, follow, perceive, make out, penetrate, divine, search out, ferret out, puzzle out, take in, assimilate, absorb, get to the bottom of;
interpret, decipher, decode, disentangle, untangle, unravel, piece together;
informal make head or tail of, take on board, get a fix on, get/catch the drift of, tumble to, crack, dig, get, get the picture, get the message, see what's what;
British, informal twig, suss (out);
North American, informal savvy;
rare cognize.