Hi Inês,
The typical thing to do is store a local copy inside the block of that particular input parameter you're interested in, and in the OnParametersChanged of the block, add an If node comparing the input value to the local value. And of course after that, updating the local value with the input value.
Getting that information in some way from the platform rather than from your own logic, would take away the need to have a local variable and to keep that variable up to date, but you will still have that If.
In the meantime, if you have large blocks with lots of inputs, and you don't want to clutter up your OnParametersChanged action with all kinds of If's and assigns, and don't want to clutter up your blocks parameters with lots of local copies of parameters, here's a workaround :
You could defer the comparison and keeping local copy to a helper block, and avoid the If by having a having a handler in your block to handle only the change of that specific input variable.
Can't share an oml in ideas section, so not sure if I can explain properly :
In it's simplest form, you just have a helper block, let's call it ParameterChangeMonitor, that has a single input text variable, only an If False on the canvas, with something in the true to make it visible in SS, an OnParametersChanged action that launches a parameterless event.
In the block where you are for example interested in some code only happening if inputX changes, you put an instance of ParameterChangeMonitor in your block, pass inputX into it, and handle the event of that block in a handler called for example "inputXchanged"
You could give it more features, like having the event pass back old and new value and things like that.
Dorine