The onus of proof is on Iran... not the rest of the world. The onus is on Iran to prove that its nuclear program is not being use for the development of a nuclear weapon. They need to stop enrichment as per UNSC resolutions, sign the Additional Protocols, open up their sites to immediate inspection. If they don't comply a total blockade should be imposed. If Russia and China don't come on board then it's military action.
Riot police line up as fresh migrant protests erupt at Budapest railway station (Reuters)
from the article: Report: New Iran nuclear facility will be operational soon