It is hard to imagine a situation where you would need to determine
which is topmost.
I have a Master Form, calling different Slave Forms. One of the Slave
Form detects
missing values required for processing und want to act as a Master
Form calling now
the Slave Form responsible for the input of the required values.

If it is a problem, it sounds like a design problem
Ok. I reflected your hint and redesigned it top bottom:
The Master Form itself checks now for the missing values required
for it's Slave Form responsible for the processing. If the values are
missing the Slave Form responsible for the input is called and in
the following the Slave Form responsible for the processing at last.
Addresses of the calling forms are passed via a public property as
a centrral Interface.
Hope this one helps. Tests are outstanding

vbr
ChFa