How do I test if my DX5e antenna is still good?

That's probably OK, but you should do a range test.

All transmitters have a mode that transmits a much lower power than normal, and if that is sufficient to control the model at 30 paces, full power will be enough to fly further than you can see.

From page 9 of the DX5e manual:

2. Face the model with the transmitter in your normal flying position. Pull and hold the trainer switch while toggling the HI/LO Rate Switch four times. The LEDs will flash and the alarm will sound indicating the system is in range check mode. note: The trainer switch must be held the entire time during the range check process; releasing the trainer switch will exit the range check mode.

3. You should have total control of the model with the trainer switch pulled at 30 paces (90 feet/28 meters).

Now an Inductrix probably won't have the range of a normal receiver so don't worry if it's a little short, but you should range test every receiver you buy every time you install it in a different model. Ideally from a range of angles to make sure the antenna can't be hidden behind a large metal or carbon-fibre part.

Before doing a range test, it's important to either remove the props or ensure that the model will shut down if it loses signal, to prevent it flying away.