Item check param

How can I make an item which increase a param of an actor (but only if that param is under a certain value, otherwise it has to show a message)?

have the item trigger a common event as an effect, where you'll check whatever you want before doing anything.
You'll have to use one of the tricks for identifying the item target however, either with a state or with script commands.

Thanks, I have found something to check the param, but I have no idea how I can check the item target...(sadly, I'm unable to script)
Can you help me?

make an invisible target state and have that state applied to the person using the potion as an effect.
in the common event, test for every actor if he has the state - if yes, check its data and remove the target state at the end.

tedious, especially with a lot of actors, but the simplest non-script method.