To articulate your assumptions:
Start with historical data. Even when times are changing quickly, information about past performance can establish a base to start from.
Trust your own experience. Make educated guesses about what’s likely to happen.
Listen to your intuition. Even though you can’t verify those “gut feelings,” you can take them into account.
Conduct due diligence. You may have to do your own research, read trade journals, collect industry statistics, and so on.