I'm assuming you might want to read from Archive. If so, this code should work: (Note: requires 9 bytes of RAM, here assumed to be at L1)
21->{L1} .The 21 means appvar
Copy(input,L1+1,8)
If GetCalc(L1,Y1)
.Variable exists, Y1 is the relevant file
Else
.Variable doesn't exist
EndI'm gonna test this, but it ought to be good.
Edit: For posterity: this code is wrong, mainly because I forgot how input works. It often gets the right result, but it's a coincidence that it does.

Right, and you can also do it by modifying some data in the program. (Note that only needs to Buff(8 ), though, and there's no need to pre-fill it with zeros)
Edit: Tested and works

Nope. The string pointed to by input is null-terminated, which is copied over (unless it's 8 chars, but that's okay) and that's how GetCalc( reads it as well. And if the name is 8 characters long, it doesn't need a null.
Edit: Tested it like this; it worked:
21->{L1
Fill(L1+1,9,85 .Garbage
For(X,1,2
Copy(input,L1+1,8
If GetCalc(L1,Y1
Disp "HURRAY
Else
Disp "AWW...
End
EndThe names, in order, were ASAVEPIC and DCS7.
Edit: For posterity: this code is wrong, mainly because I forgot how input works. It often gets the right result, but it's a coincidence that it does.

Nope. The string pointed to by input is null-terminated, which is copied over (unless it's 8 chars, but that's okay) and that's how GetCalc( reads it as well. And if the name is 8 characters long, it doesn't need a null.

Actually, doesn't input return a pointer to the data section of a temporary variable? It's preceded by a two-byte length, and likely not zero-terminated.

Documentation says pointer to the "string structure". You're probably right it's an OS variable. Oops

All right, fixed code time. Now requires 10 bytes of RAM at L1 'cause I don't feel like special-casing the length-8 case.
21->{L1}
Copy(input->P,L1+1,min({P-2}r,8))
Asm(EB3600) .Puts 0 at the end. Assumes DE=Y+L after Copy(X,Y,L). Else just do 0->{L1+1+L}, where L is stored from the call to min(
If GetCalc(L1,Y1
.Exists
Else
.DNE
EndEdit: Works, and for the right reasons this time.