Objects in Hebrew are usually marked out by the word את. video So in Genesis 1:1, we know that הַשָּׁמַיִם is the object of בָּרָא because of the object marker.
בְּרֵאשִׁית בָּרָא אֱלֹהִים אֵת הַשָּׁמַיִם וְאֵת הָאָרֶץ׃
Many times, the object marker word will be connected to its noun with a maqqef as in Gen. 1:4.
וַיַּרְא אֱלֹהִים אֶת־הָאֹור כִּי־טֹוב וַיַּבְדֵּל אֱלֹהִים בֵּין הָאֹור וּבֵין הַחֹשֶׁךְ׃
This object marker word not only marks out the object but also marks that noun as definite. This word can also take pronominal suffixes (KHW 23.2b; see chp 9(p) here).
Note: there is a preposition which is spelled exactly the same; see here or Gesenius §103(1). The way to distinguish is to note that the object marker word will take an O class vowel, while the preposition will take an I class vowel. See chp 9(p) here.
Thus all nouns used in connected discourse are definite or indefinite, either from their own nature, i.e. in consequence of the meaning in which they are used; or from choice, i.e. through their assumption of the article: and every substantive in a sentence must necessarily be considered as standing in either of these two relations. But this variability in the condition of nouns, whether as definite (through assumption of the article, or in virtue of their own meaning) or freely indefinite, is of very great importance and significance; because it must also exert a reflex influence on the surrounding words, and because a definite noun, especially one which is necessarily such, has much more weight and force in a sentence than one which is not defined. This contrast will be found to reach through a large portion of the sentence, and with it there is associated the equally important antithesis between a noun which indicates an animate, and another which signifies an inanimate being; for, though a definite noun is always of more importance in a sentence than one which is indefinite, what is animate is likewise considered as of much greater importance than that which is inanimate, and hence also as having more need of being distinguished by an outward sign. Several modes of indicating such words have already been treated of elsewhere; but a mark of peculiar importance in this case is found in the use of אוֹת or אֶת as the sign of the accusative. source