[How do I tell if the capacitors are going bad?]

I just bought some model 17's off of Craigslist. Pretty beat up but sound fairly good. At times the tweeters sound a little fuzzy. Is this a characteristic of these or is something amiss. I know the caps tend to go bad in these. I have not had a look inside yet, but how do I know if the caps are bad or in need of replacing?

Thanks.