Who can discern the face of his garment? - literally, "Who can reveal the face, that is, the appearance, of his garment?" This "garment" is undoubtedly his skin. The meaning seems to be, "His hard and rough skin is his defense, and no one can so strip off that as to have access to him." The word rendered "discover" (גלה gâlâh) means "to make naked"; then "to reveal"; and the idea is, that he cannot be made naked of that covering, or deprived of it so that one could attack him.
Or who can come to him with his double bridle? - Margin, "within" Gesenius renders this, "The doubling of his jaws;" that is. his double row of teeth. Umbreit, "His double bit." Noyes, "Who will approach his jaws?" So Rosenmuller. Schultens and Prof. Lee, however, suppose it means that no one can come near to him and "double the bit" upon him, "i. e." cast the bit or noose over his nose, so as to secure him by doubling it, or passing it around him. The former seems to me to be the true meaning. "Into the doubling of his jaws, who can enter?" That is, Who will dare approach a double row of teeth so formidable?" The word rendered "bridle" (רסן resen) means properly a curb or halter, which goes over a horse's nose, and hence, a bit or bridle. But it may be used to denote the interior of the mouth, the jaws, where the bit is placed, and then the phrase denotes the double row of teeth of the animal. Thus, the description of the "parts of defense" of the animal is kept up.

Who can discover the face of his garment? - Who can rip up the hide of this terrible monster? Who can take away his covering, in order to pierce his vitals?

Who can discover the face (d) of his garment? [or] who can come [to him] with his double (e) bridle?
(d) That is, who dare pull off his skin?
(e) Who dare put a bridle in his mouth?

Who can discover the face of his garment?.... Or rather uncover it? Not the sea, which Mr. Broughton represents as the garment of the whale; who can strip him of it, or take him out of that, and bring him to land? which, though not impossible, is difficult: but either the garment of his face, the large bulk or prominence that hangs over his eyes; or rather his skin. Who dare venture to take off his skin, or flay him alive? or take off the scaly coat of the crocodile, which is like a coat of mail to him, and which he never of himself casts off, as serpents do?
or who can come to him with his double bridle? either go within his jaws, which, when opened, are like a double bridle; or go near and open his jaws, and put a curb bridle into them, and lead, direct, and rule him at pleasure. This is not to be done either to the whale or crocodile; yet the Tentyritae had a way of getting upon the back of the crocodile; and by putting a stick across its mouth, as it opened it to bite them, and so holding both the ends of it with the right and left hands, as with a bridle, brought them to land, as Pliny (s) relates; and so the Nereides are represented as sitting on the backs of whales by Theocritus (t).
(s) Ut supra. (Plin. l. 8. c. 25.) (t) Idyll. 19.

discover--rather, "uncover the surface" of his garment (skin, Job 10:11): strip off the hard outer coat with which the inner skin is covered.
with--rather, "within his double jaws"; literally, "bridle"; hence that into which the bridle is put, the double row of teeth; but "bridle" is used to imply that none dare put his hand in to insert a bridle where in other animals it is placed (Job 41:4; Job 39:10).

Discover - Or, uncover, or take off from him. Face - The upper or outward part of his garment, or, the garment itself: the word face being often redundant. And by the garment is meant the skin which covers the whole body; who dare attempt to touch his very skin? Much less to give him a wound. His double bridle - His fast jaws, which have some resemblance to a double bridle: whence the Greeks call those parts of the face which reach to the jaws on both sides, the bridles.