If you know you will have a very linear progression of forms, MichaelRed's suggestion will work, but here is a scenario where it doesn't work:
Open Form A
Open Form B from Form A
Open Form C from Form A (without closing Form B)
The stack now holds
Form A
Form B
Form C
suggesting that Form C was opened from Form A
Also, if you want to know what form called Form B, you can't just look at the end of the stack - 1, you have to do a search for Form B.
In the code I gave you above, you just use frmCalledFrom within Form B to find the calling form. Also, I don't know why you want the name of the calling form, but I'd guess you want to do something with it, hide it, show it, close it, whatever. That would indicate you're not really looking for the form name, but for a pointer to the form, which frmCalledFrom gives you.