Learn German: Eine neue Maßnahme ist heute zu planen. - A new measure is to be planned today.

This uses the sein + zu + infinitive structure, which acts as an alternative to the passive voice. It expresses that something either must be done (necessity) or can be done (possibility). In this sentence, it means the measure needs to be planned.

How would I say this using a standard passive sentence with a modal verb?

You would say Eine neue Maßnahme muss heute geplant werden. The sein + zu structure used in this sentence is simply a more formal and concise way to express that same necessity.

How do I know if ist ... zu planen means 'must be planned' or 'can be planned'?

It comes entirely down to context! Here, scheduling a measure sounds like an obligation (necessity). If the sentence included an adjective like leicht (easy) or schwer (hard), it would usually express possibility, like Die Maßnahme ist leicht zu planen (The measure is easy to plan / can be planned easily).

Why is the adjective ending -e on neue?

Maßnahme is a feminine noun in the nominative case here, because it is the subject of the sentence. After the feminine indefinite article eine, the adjective takes the standard -e ending.