In an InputScript, how can I compare the field value before input with the field value after input?
Use a "Link" command and an additional variable to save the value before input. Example:

// GuiXT Script

Set V[Amount_old] "&F[Amount]"

Link V[Amount_new] F[Amount]

// InputScript

// Amount changed?

if not V[Amount_new=&V[Amount_old]]

// action on value change