This is a tough question to answer, because it assumes we all agree on what true love is, and which characters in the play feel it. To be honest, I'm not sure any characters in the play feel true love except for perhaps Theseus and Hippolyta, and their own love is not threatened during the course of the play.
The next closest to true love would be the two pairs of young lovers at the start of the play. Both of them react with strong commitment when their love is threatened. Hermia, for example, is threatened with death or imprisonment if she does not marry Demetrius, but she refuses and stays true to Lysander.
So, they react by hardening in their love; you might say that threats show whether love is true or not by making the threatened love stronger.