From what is being reported there will be very limited CM previews on 4/25 with full CM previews on 5/2 followed by the AP previews on 5/10-5/12. Soft openings are possible around those times but not guaranteed. Even if the current plan is to have the attraction open during those times (not sure if it is but it is pretty common for a new attraction) it can close at any time.
Soft openings are never announced or known even day of as CM would hate to say it will be open and then it will be closed for whatever reason. However, with that said I would still ask a CM around the area as they can give you the best idea that soft opening previews have happened previous days. I know when MILF was in soft previews we asked a CM and they said the best time to try is after the 3 PM parade as that is when it has been done previously. Even asking trips after for MILF the CM would say they had no clue and then 1 hour later the same CM would be directing people in, it is understandable though that they don't know and they don't want to set anything as when they do people will not understand and be ****ed. Good luck!
No, all you can do is go by the attraction and check and even then the CM's at the attraction can not tell you if or when it will be open. If they inform the general public of a soft opening they run the risk of hundreds of complaints if and when they need to shut the ride down for whatever reason. That unfortunately is the nature of soft openings.
For example on the first day of the PotC soft openings CM's spent a good hour telling people over and over again that the attraction was not open, do not waste you entire day waiting for something that might not happen, etc, etc. The ropes dropped a bit after 10 AM and the ride opened but it could just as easily not have opened at all.