If cognac is marked kosher for Passover by a reliable certifying agency, you can rest assured that it is just that. After all, the base ingredient of cognac is grape, which is kosher for Passover. However, since spices and other flavors are often added to the mix, you cannot be sure that cognac (even if it is marked kosher) is kosher for Passover, unless it bears Passover certification.

As you can read here, many in Chabad traditionally avoid all alcohol on Passover (with the exception of wine) as well as many spices, and this would presumably mean that cognac is off the Passover menu, even if it is certified kosher for Passover.

It is also very important to note that as a wine-based product, cognac is subject to a unique set of kosher laws and must be certified kosher, even for year-round use.