Jack,
This is 32 bit MASM format but it shows the basics of how to use these APIs. The "lpszFileName" is simply the address of the file you want to test. Check in 64 bit if EAX needs to be changed to RAX based on the WIN32_FIND_DATA structure.
exist proc lpszFileName:DWORD
LOCAL wfd :WIN32_FIND_DATA
invoke FindFirstFile,lpszFileName,ADDR wfd
.if eax == INVALID_HANDLE_VALUE
mov eax, 0 ; 0 = NOT exist
.else
invoke FindClose, eax
mov eax, 1 ; 1 = exist
.endif
ret
exist endp