If people are still leaving, then it's likely that they don't see job security in their current positions -- not surprising, considering the downsizing. I think the first step would be to talk to the team and tell them that things are getting better and that downsizing is over -- many companies overlook this small but crucial piece of communication.
To motivate them to stay, if you know things are going to improve in 1-2 years, you can share that information with them -- whatever little you know. That's the most you can probably do. You can also personalize with them, and tell them that your salary is as frozen as theirs is. I had one manager tell me this, and it did make me feel better.
Asking them if they're looking is tricky; they may lie or deny it, or may feel insecure even if they tell you. It really depends how much they trust you. If they think you may hold it against them, they definitely won't even breathe a hint of it. It also depends how much you trust yourself; will you really treat them the same if you know they're looking to get out?
I would say, if you're really going to be bold and ask them, do it in a smart way: create an anonymous employee satisfaction survey (Google Docs can do it) and ask them if they're leaving, and put an optional field for their name. You can explain your reasons for wanting to know. If they want to tell you, they can; but you're not forcing them to tell you anything they don't want to.
Finally, for managing risks, you probably already have a risk register with a list of risks and their probability and impact (if you don't, make one ASAP!) If it's not used by the team, you can add individuals or people of certain roles leaving (eg. senior developer); if it is shared, make a private version to manage these risks. This is probably not something you want to tout publicly.