Truth Compulsion – How to Force a Character to Tell the Truth in D&D?

Funnily enough, works by RAW.

There is no prohibition of magical healing in construct traits, though some constructs are explicitly immune to magic except select few effects.

There is also no prohibition in the heal spell.

Technically, you can do it, but expect a heavy book thrown by your DM. He will also probably houserule it away, at least I personally would.

Note:

Can't remember anything on that in 3.5 either, though the description of Warforged contrasts the possibility of both special and positive-energy healing (emphasis mine):

As living constructs, warforged can be affected by spells that target living creatures as well as by those that target constructs. Damage dealt to a warforged can be healed by a cure light wounds spell or a repair light damage spell, for example, and a warforged is vulnerable to disable construct and harm. However, spells from the healing subschool and supernatural abilities that cure hit point damage or ability damage provide only half their normal effect to a warforged.

That leads one to the implication of impossibility of positive-energy based healing for a regular construct, even if it is not stated anywhere.

The rules are silent on this.

In practice, I've seen it both ways, depending on whether the group takes a more gamist/metagamey approach or a more simulationist approach. "Open rules" games just have the GM tell you "that fails" if it's invalid on its face (only concealing the reason if it plays into the challenge, like "Charm Person fails... I wonder what that guy is if not a humanoid?" But in general you roll for SR, roll the save, you know why it failed. In our current Pathfinder campaign the GM runs it like this, and whenever it's in doubt a flurry of Knowledge checks fills in the gap.

In a more sim game those things are kept more "under the table" because it's not about the rules challenge it's about the in-game-world reasoning challenge. I ran a 5-year 2e campaign that worked like this.

This falls into the lacuna of other things like "do I know if I made a save vs. a spell" that is undefined so that different games can do it differently based on their desired feel.