title_body: Is there any way that I can discern the IBU of a beer if the bewer doesn't provide that information?
upvoted_answer: I would argue that even having the IBUs on the bottle you still don't really know what you're going to get.
  
  The bitterness of the beer isn't just down to the quantity of alpha acids, which is what the IBUs measure. The bitterness is offset by any residual sweetness in the beer.
  
  A brewer can brew two different beers with the same IBUs, and them taste different. For example, with two beers with 20 IBUs - one might taste balanced between sweet and bitter, while the other one just tastes sweet. Another pair of beers with 70 IBUs, one might be unpleasantly bitter, while the other a more balanced kind bitter, or even simply neutral. The FG can give you some insight into these differences, but it's still not going to be precise.
  
  Frankly the best way to know how the bitterness of the beer plays with the rest of the beer qualities is to read the label for any subjective descriptive text, or simply buy it and taste it. 
  
  If you did want to measure it, you would need to use spectrophotometer and solvent extraction.