If a member of law enforcement says that they are going to confiscate your knife, and if you know that you are carrying a legal knife in a legal manner, you can try the following-
1. Politely inform the officer that you know the law and cite the specific penal statutes that make your knife and method of carry legal.
2. Inform them that you have in your possession a printout of your local penal code to show them (you may need to be carrying this in order for it to work).
3. Request that the officer call their superior officer on the street to the location to verify the law.
4. Inform the officer that the first thing you are going to do if the cop takes your knife is call your civil attorney and have him meet you at the police station.
Of course, none of this provides any guarantee of success. And your attitude might also play a significant role in the decision of the officer or officers. Some cops simply don't know the law, some might assume that you don't know the law and try to take advantage of you, so politely informing a member of law enforcement that you do in fact know the law might help you keep your knife.
If after all your efforts at the scene, a member of law enforcement insists on confiscating your knife, then there isn't anything you can do about it at that moment. Obviously, any physical resistance on your part will likely be met with pepper spray, a taser, a baton, or gunshots. And you will end up in either the morgue, the hospital, or jail. You can't win an argument with a cop on the street. And you certainly can't win a fight with a cop on the street.
If a cop does insist on confiscating your knife, there are things you can do that might help you get the knife back. Here's a list-
1. Ask for a receipt.
2. Get the cops name and badge number and station location (as well as the names and numbers of any other cops at the scene).
3. Get the license number from the plate on the cops car, as well as any other identifying numbers painted on the car (as well as the plate and numbers from any other cop cars at the scene).
4. Right down the exact time and location when and where the knife was taken.
5. Get the names and phone numbers of any witnesses.
6. Ask any local businesses if they have surveillance cameras that might have recorded the incident.
7. Visit the station and politely request your knife back.
8. Contact a civil attorney.
If you are going to file a complaint, or a civil claim, against a member of law enforcement, any such information might help you. If you are going up against law enforcement, or the legal system, there is no such thing as having too much information on your side.
Here are a few other good ideas-
1. Keep quality photos of your knives. And if one of your knives is confiscated, you can bring a picture of the knife with you to the station to show exactly what the knife looks like. Such photos might also prove valuable if the matter becomes a civil case.
2. Engrave your knives with your personal information. This can make it harder for a cop to lie and claim a knife in his possession isn't yours if it has your name and address engraved on it.
Of course, there are no guarantees when it comes to getting your knife back. And if members of law enforcement are determined to conduct themselves in a corrupt manner, and unless you are wealthy, or have political influence, you are likely going to get screwed.
However, speaking personally, if any piece of my property were unlawfully taken by a member of law enforcement I would use every possible means at my disposal to get it back, and if appropriate, to see that officer punished. Some people in our society are of the attitude that you should never make trouble with the cops, that if you complain or sue that they will make your life hell, but I strongly disagree with that attitude. If you aren't willing to stand up for your rights, don't be surprised if they are violated. When citizens keep quiet about police misconduct out of fear or for the sake of convenience, they only encourage more police misconduct.
And as always, when it comes to legal matters, I recommend consulting with an attorney.