They are doing an AP Preview Day on March 4th, so it's very unlikely that it would be open to the general public on that day. In terms of a soft opening on any other day, DLR does not announce soft openings. Basically, you just have to be in the park and happen to be there when they're allowing guests to ride the attraction. They won't announce it, nor is there any guarantee that even if you hear it's open, that it'll be open for the entire day. Basically, for any soft opening, it's just a matter of luck.
I found it curious the AP preview is Friday, March 4th and the general public opening is Monday, March 7. Nothing I saw scheduled for the 2 days in between. Perhaps they'll do something over the weekend...