To be successful you've got to be able to hear either the lowest or highest note in the triad, and then identify the intervals above or below it (P4 and M2).

Sounds obvious... How do you get there... Try these:
- Make sure you're good at identifying M2 and P4 intervals rising, falling and harmonic
- Make sure you can sing M2 and P4 intervals rising and falling
- When you're identifying M2 and P4 intervals, whichever it is - try adding the other on top. E.g. if ToneGym throws a M2 at you, try adding a P4 above the upper note or below the lower note
- Work on Inversionist and sing all the intervals. Even identifying intervals in other triads/chords will improve you're ability for sus chords

You can do a load of other stuff, these are just starters. Oh and have fun!!

I was just thinking about this. @Miles Walker is on the money. Learning to hear M2, P4 and P5 interval helps a lot.
Do you hear the M2 on the bottom and a P5 on the top? It’s probably SUS2. If you hear the M2 on the top and P5 on the bottom it’s SUS4.
Like you hear a clean note on top or bottom and a dissonance M2.

Don't choose the Xylophone.

To add to Miles' answer, I sometimes hear a little resolution figure in my head. In root positions, if it sounds like something (the middle pitch) wants to resolve up, that's a sus2. If it sounds like it wants to resolve downward, or you can imagine a little plagal cadence (foooouuur-three-two-threeeeee), that's a sus4.