I read that there is a Denver-specific AWB. wiki link.
What are the limits of Denver? I have a friend moving to Aurora.... is that somehow considered part of Denver?

I saw this pin, but it's locked. How do you prove you owned a high-cap magazine prior to the change in the law?

The City and County of Denver think they are special and that they can make their own set of firearm regulations regarding assault weapons and magazine capacities.  They believe pre-emption does not apply to them, claiming home rule.  IDK if they have been challenged on this, but as of now, I don't think so.  They were challenged on peaceable journey and lost.

The burden of proof is on the government to prove when you owned the magazines.  If the magazines have no date stamp, or if it "rubbed" off, then you have no idea when they were manufactured, let alone purchased.  If there is a date stamp, you would still have to prove when possession took place.  Good luck prosecutor.  Bill sponsors admitted during debate, this would be very a very difficult hurdle to clear.  The only way you could be caught is if the date stamp were post July 2013 (the date the law took effect).  

Only one person has been charged for a magazine, and it was an add on to a list of other crimes.  I think that  it was eventually dropped.

This is all from memory, I'm not an attorney, read at your own risk, etc......but......there are stores openly selling standard capacity magazines.  Many (if not most) county sherrif's offices have stated they will not enforce the mag ban.

ETA:  Aurora isn't in Denver county.

FWIW, I don't care what the limits are in Denver.  They can KMB.

https://www.google.com/maps/place/Denver,+CO/

https://www.denvergov.org/property

What is preemption?

Check your facts
http://www.krdo.com/news/colorado-springs/fact-check-colorados-gun-magazine-ban/323772960

Kind of like how states ignore the Bill of Rights/2A and make their own laws? How many levels can it go down... Can I preempt federal, state, county... oh wait that's what the Bill of Rights is intended... inalienable rights of the individual. Wow, it just went full circle... and I melted my brain.

I like this

"What we have done is make criminals out of law-abiding citizens.  And quite frankly, the criminals still get whatever magazines they want," explains Republican State Senator Jerry Sonnenberg of Sterling.