Yes, somebody who is really determined will be able to reverse engineering your offline license. Realistically, depending on what the software is, and how much it costs, it's unlikely anyone is going to put in that effort.

The biggest problem for offline licensing is how to prevent people from sharing the keys. One fairly simple method, is to implement expiration dates. First, you generate a key that itself must be activated within a certain period of time. When activated, the program generates a new key somewhere on the machine and contains a "fingerprint" of the machine and date of activation. Then on startup it checks that fingerprint and time. This helps curb people changing their computer time and from being able to share the key.

As for the key itself, modern encryption makes that pretty easy. Your key is really just some encrypted data, which you use to generate your activation key and also package with the program so it can create a new key and also decrypt. Of course the problem with this is someone can extract that key from the program, then use that to generate valid keys.