The standard answer I give is that other flavorings can be used besides for the fruit, and the fruit can be used in many forms such as purees or zest, and can be questionable from a kosher standpoint.
The following is an actual email I recieved from someone who contacted me at the email address listed on this site. The original email asked me about a beer, that is made with raspberries and therefore should be acceptable even without proper certification. I responded that as per Rabbi Niehaus from the CRC one can be their own mashgiach on a beer, by inquiring to the brewery and try to ascertain the origin of the ingredients.
Below is his response:
I would like to thank the individual for allowing me to use his email, and I will respect his request for anonymity.
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