I know a lot of people on here are going to take the high road and say don't lie, etc. But lets talk about reality and the reality is this is a negotiation, no different than negotiating the purchase of a car from a car salesman. You do not have perfect information. One of the areas that you lack information is will this interviewer use the knowledge of my former salary against me. Not only is it not uncommon but it is pretty usual. I would say (back when I used to work for other people) 7 out of 10 times I would be put in a situation where the interviewer was trying to leverage me down based on knowledge of my former salary. I used to feel that this was unethical for them to do until I understood that you are in a negotiation and for many the standard rule of ethnics does not apply. You are simply trying to get the best deal you can out of the situation using as many variables as you can. So given the situation you have one of 2 choices lie to offset the possibility that this interviewer will use the information to leverage you down, this way you are starting form a false ceiling. Or secondly refuse to divulge that info with an explanation that you feel it would be used against you, that you have an aversion to lying and don't want to be forced into a situation to lie to defend your ground in negotiating. Many interviewer do not get that form of response and some may be impressed by your principals you could very well frame the situation out of a negotiation by doing so, you also run the risk of aggravating the interviewer if they press for the info and you stand firm.

So ethically what do you do, well I always like to frame the situation as a car sale, if a car salesman asked well how much do you have to spend on the car. What is your answer is it naturally less that the sticker on the car. You see the sticker price is very much like your previous salary, but the car lot has chosen to inflate it to give themselves wiggle room. Ethically few have a problem with this even though almost every line item on the factory invoice is a lie. It is common knowledge that they are inflated. Further if asked how much you have to spend on the car most answer somewhere below the sticker, trying to move that number down and reduce some of those inflated items to somewhere closer to reality. Finally the salesman hold the knowledge as to the true value of the car but would never offer it over as a tool in negotiating. This would be akin to you original non-inflated former salary.

Looking at that hypothetical situation, flipping the parties and reversing the roles you can see the ethics involved. Every modern corporation works this way and one would be well served to remember that in the end they are dealing with corporations. We as a society have a double standard in which corporations are permitted to have dubious ethics as standard operating procedure while the individual is lambasted for items as small as inflating ones salary so they don't get cut to the bone in salary negotiation.

For me personally, I see no issue with inflating a former salary, but that is because I am a skilled negotiator, I don't take it personal and I expect disinformation. In a weird way I am impressed when someone bests me at the negotiating table and most of all, I understand that the person at the negotiating table is fighting for their scraps what they do at the negotiation table has little reflection on what their true character is. If I found out that an employee lied to me about a former salary, I would not terminate them, because I understand that culturally corporations use this information all the time to leverage them down. They are paranoid about it and rightfully so.

As for them finding out, I don't know where you are at in the world, but in the US their is no way for them to find out unless you provide them with that information. They cannot ask for that info from a former employer and no employer would open themselves up to the liability of providing that info to a third party and they cannot access any tax info on you. So short of you providing it you should have little worry of them finding out.

On a side note, I did have an potential employer, as part of their application process, require that I submit two pay stubs from my last job. Their excuse was to prove that I had been employed. I know otherwise but anyways, I knew providing this info was basically giving up my seat at the negotiation table so I told them, that before I would provide that info that we negotiate salary and that if they tried to renegotiate my salary based on that info that my salary requirements would go up by 25%. So I went and interviewed, with them and the client that they where going to place me at, the client loved me, and wanted me to start ASAP so I had to get my paperwork in order. So I sen it all over and wouldn't you know, all the sudden they where not able to meet the rate that we negotiated and it just so happened that they rate they could meet was about 7k over my old rate. I hit back with the 25% increase, stood firm for a few days, the client was chomping at the bit and they where just trying to get me back at the agreed upon rate. About a week later another offer from a different company came in for what I was originally looking for, I took that job and left the others to deal with the ramifications of their actions.

The point is employers will use this info against you even if they say they wont. You have to ensure you look out for yourself when negotiation for salary. You are in direct opposition to the companies interest.