|Verúmtamen óculis tuis considerábis, ..||But you shall look with your eyes, ..|
verumtamen = but.
oculis tuis = with your eyes. From oculus,i: eye. Oculis could be dative or ablative (plural). Here it is ablative. How do we know? From the context. Specifically this is an ablative of means.
Suppose you want to express in Latin the sentence, “She hit him with a stick,” using baculus,i for stick. You could write, “Ea percussit eum cum baculo.” Cum is a preposition meaning with. But it would also be proper, and perhaps more stylish, to use the ablative of means. You put the word stick in the ablative, and it becomes the thing by means of which the verb is accomplished: “Ea percussit eum baculo.” She hit him with a stick, or by means of a stick. And so the psalmist is saying that you shall look by means of your eyes, as opposed to a figurative way of seeing (such as understanding or taking someone else’s word for it).