How to prove that I have worked on complex product design in the past?

The only proof you need is that you can talk in detail about the design and the issues that you encountered with it. At the interview you should be using previous projects to provide examples of how you solved problems or used your knowledge and skill. The fact that you can confidently and clearly explain those things, with a true narrative showing the process, is all the proof you require.

Of course if they do offer you the job they can contact your references to get confirmation that you worked on the projects you say you did. Make sure your references are aware of your contributions.

If the interviewer demands more proof than that you can only explain that copyright makes it impossible, and then realize you dodged a bullet anyway because you don't want to work for a boss who doubts your efforts.

I assume that your work eventually turned up in some product that I could buy in a store For little money or that my company might buy for an awful lot of money. So if it is not a secret that your company produced parts of these products, you could say that You developed parts of products A, B, and C. If product X was secret, you add “and one other product that is under NDA”.

You can ask your company if you are allowed to send photos of the hardware. Most likely I could get the hardware itself by buying the product and taking it apart, so your company may not mind. Or if they have brochures that anyone would receive from them, you could ask for them and mark what you specifically designed.

Do you need proof that you are designer? If I received this material, I’d say “he is either really good, or a blatant liar”. Anything in a CV could be made up, we all know that, so it’s no problem. You’d most likely get the interview.

PS. Anything that I as a random person could view on your company’s website you can obviously mention. Include a link on your CV.

It all depends on the person interviewing you. But you must remember that businesses know that information is confidential, so they will not require you to break confidentiality.

How will the new (potential) employer that you do not lie about the work done? It is simple - you only need to talk about it, describe some of the aspects which matter, without being secret. Depending on the way you talk, it will be (relatively) clear what kind of job you did.

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Example: how will anyone believe you that you did some great job, if you do not know the initials P.C.B come from. Or why are there small holes in that green / red / brown / whatever-color board.

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Therefore I feel like it would be very useful to be able to show potential new employers a few photographs of some of the equipment that I have created in the past.

Merely taking confidential information outside of the company's premises is already very dangerous, if you do not have the proper written approval to do so. Showing the said info to third parties, especially other companies, only adds to the danger.

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I work in international corporations for about 20 years, I changed jobs several times. I never showed anything during interviews (except the CV). I explained everything that was asked, without giving away confidential information. I never needed to look much for a new job.

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Alternative (what I do)

Occasionally, I wanted to show pictures related to what I worked in the past. And I did, safely.

I just go to the official site of the company and search there the products that I worked on. I do this usually for the products which are not know to the wide public. I just use the information already made public by the company.

Or, I tell the people the names of the products for which I worked (if the products are known to the people). Usually I disclose that information only after the products were released, or at least presented in some shows. After that, anyone can search any information they want.

If you are planning to use the photos as a conversation starter in your interview. See it as a crutch to set you at ease and to initiate a conversation about your work history.

As you say yourself you are not planning to work for a competitor or in the same sector.

You can explicitly mention that it is early high-level designs or prototype and not the final product perhaps.

As an aside ensure you describe these designs in detail on your resume/c.v as it showcases your written communication skills. A good communicator is a skill employers will be looking for, especially if you want to progress to a more senior position.