You can estimate bitterness by comparing it to a known hop by making a hop tea with the same amount of each hop, and then adding a measured amount of sugar to each tea, tasting and repeating until the sweetness is in balance with the bitterness.

The unknown hop will have an approximate AA% related to how much more or less sugar it took to counteract the bitterness vs the known hop. So say your known hop is 5% AA, and the unknown takes 50% more sugar to counteract the bitterness. It would be approximately 7.5% AA. If it took 80% as much sugar, it'd be approximately 4% AA.

These are very rough approximations, but close enough to get you in the ballpark for brewing.

Edit: Found a link to an article that explains the process better. It's towards the bottom of the page.