What gave it away? verse 2?
A lot of verses "gave it away", making it clear. One was verse 1:6, as pointed out in the OP:
And God said, “Let there be a dome in the midst of the waters, and let it separate the waters from the waters.” So God made the dome and separated the waters that were under the dome from the waters that were above the dome. And it was so. God called the dome Sky.
A good discussion of this from a Christian point of view is here:
http://www.christianorthodox.net/beyond-firmament-thinking/
There are lots of other verses too, of course:
Flat Earth-
Bible tells us that the earth is flat like a piece of clay stamped under a seal (Job 38:13-14), that it has edges as only a flat plane would (Job 38:13-14,.Psa 19:4), is set on a foundation, like a table (2Sm 22:16, Ps 18:15, 102:25, Pr 8:27-29, Is 48:13), has a length as only a flat plane would (Dan 4:11, Job 11:9, Job 28:24, Job 37:3, Job 38:13, Job 38:44, Jrm 16:19), that it is a circular disk (Isa 40:22), and that its entire surface can be seen from a high tree (Dan 4:10-11) heaven (Job 28:24) or mountain (Matt 4:8) or which is impossible for a sphere, but possible for a flat disk. Taken literally, as the YECs insist we do, any one of these passages shows a flat earth. Taken together, they are even more clear. And many Christians in history have interpreted it as such.
Geocentrism-
The Bible describes the earth as unmovable, set on a foundation of either pillars in water (1 Sam 2:8, 1 Chr 16:30, Job 9:6, 38:4, Psa 24:1-2, 75:s3, 93:1, 96:10, 104:5, 136:6). It also tells us that, although the earth does not move, the sun and stars do move about it (Josh 10:12, Psa 19:4-6, 50:1, Ecc 1:5
(note “returns”, not perspective), Hab 3:11). And that the stars could be dropped down onto the earth like fruit falling from a tree (Rev. 6:13). Taken literally, as the YECs insist we do, these verses show geocentrism. And many Christians in history have interpreted it as such.
We live under a dome-
The Bible describes the sky (firmament -- literally "metal bowl made by a hammer"- Gen 1:6-8, 1:14-17) as a solid dome, like a tent (Isa 40:22, Psa 19:4, 104:2, Pr 8:27-29, Ezk 1:26), that is arched over the surface of the earth. It also has windows to let rain/snow in (Gen 7:11, 8:2, Deut 28:12, 2 Kings 7:2, Job 37:18, Mal 3:10, Rev 4:1). Ezekiel 1:22 and Job 37:18 even tell us that it's hard like bronze and sparkles like ice, that God walks on it (Job 22:14) and can be removed (Rev 6:14). Taken literally, as the YECs insist we do, these verses show a solid sky above us. And again, many Christians in history have interpreted it as such.
In Christ Jesus-
Papias