Scientists how to prove DNA is anti-parallel?
The definitive proof came with the crystal structure of B-DNA (see Watson & Crick, 1953). Anti-parallel strands is the configuration that best explained the diffraction data while not violating chemistry (bond lengths and angles, no steric clashes, etc.).
The relevant sentence from the article is:
the sequences of the atoms in the two chains run in opposite directions
I underlined this sentence in the following screenshot of the article: