Grman Silver vs aluminum? The only way I know to test it, without damage, is to find a friend working in a lab that has a portable X-ray fluoroscope. They look like an oversize SF ray-gun. A popular make these days is Innov-X Systems Model (whatever) Spectrometer.Leaves no marks.
When I was working I got lots of nice brass stuff analyzed in the lab. And be there during the test, Lab Guys have no respect for antiques. Might file the surface down for a better reading.
Or find out what they charge to do the test. If you have it tested this way, these things have a program to tell you the name of the alloy.
DO NOT BELEIVE that program. It only knows what someone of limited knowledge programmed in it. Yes, the technician wants to believe it but don't you. Get the actual, complete analysis, then someone, me being the volunteer, can tell you what it is. Unfortunately the machine can't measure very light elements, including, aluminum so would give you a screwy answer there.
Wet Blanket again. 19th Century German Silver had a yellowish cast to it, the modern stuff is called Nickel Silver and is closer to white. The difference is another 6% zinc in the modern stuff. Unless one is a nit-picky metallurgist like yrs truly, people interchange the names 'German' and 'Nickel' Silver without paying much (any) attention to them being two different grades of copper-zinc-nickel alloy.
You may notice this when you try to replace one "German Silver" inlay on an antique rifle. Whoops . . . Keep it in the shade.