Just go with what seem reasonable. If gut instinct is too wobbly and inconclusive for you, you could work up a simple system of agility or perception rolls to determine if Character A has line of sight on Monster X, or if Character B is in range of Monster Y's breath weapon, and so on.

Without a map or miniatures, how to best determine line-of-sight, etc?

  What is the Krull dimension of $B=A[x,y,z]/\langle xy + 1, z + 1\rangle$, given $A$ is a Noetherian commutative ring? 

