Well if you want evidence, for example, there is a chinese man who has been brokering steel alloy/casings sales from CHina to Iran, and US has put him under sanctions currently and he keeps a low profile. THese type of cases tell us there is "illegal" military material or/and technology transfer from China to Iran.
While Iran is VERy resourceful and has its own domestic military technology and capabilities, its hard to believe Iran produces ALL the parts needed for modern, lighter, faster, more effective, more reliable missiles and rockets...just not possible for a developing country...all countries need international trade and transfers to produce advanced things, like missiles.
THats not my point- my point is that you should go dig deeper and you will see evidence of Missile related transfers from CHina to Iran. Remember, its UN sanctioned to transfer such things to Iran, so China wont do it openly in teh first place, but you can know whose doing it for ex by who US treasury dept sanctions. Follow the sanctions and you will find the culprits.