I have to proof a concept by safety testing / tripping some 35 Amp GFI outlets. Can anyone recommend the easiest / safest / best way to do this? Help would be greatly appreciated.

Are you in the USA or Canada? Can you tell me if this is a GFI receptacle or protected by a GFI circuit breaker.

You say a 35 amp GFI outlet. I have been an electrician in the US for over 30 years and never seen or heard of a 35 Amp outlet , or GFI. So I need some help with more details of what you have. If in the US, on the front or a lable on the outlet is a number assigned to that outlets configuration, called a NEMA number. For example, a standard 15 amp staight blade 3 wire 125 volt outlet has NEMA # 5-15R assigned to it, as a universal number, no matter which manufacturer makes it. If this is a twist lock outlet there will be an L in the number, for example L6-30R.

There are many GFI testers avalaible, but they are for the typical straight balde outlet used in every building. If you have a circuit breaker, there is very expensive test equipment available to test breakers, but not practical for someone at home, or even in a plant, to own or operate. There are testing companies that own this expensive equipment that can visit your site and test the breakers, not just for GFI trip settings, but for several other characteristics of breaker oerations.

Please provide more detail on your situation.