So yesterday I had a player inform me that I had to select a target, and then measure to that ship to see if it is a possible target. While this is true for target locks, I had always played it as if you can check range on multiple ships and then determine the target in combat when that ship is the active ship.

Where can I find this ruling or am I actually incorrect?

Rules Reference, look under Attack.

Step 1 is Declare Target

"The attacker may measure range to any number of enemy ships and check which enemy ships are inside his firing arc."

Thank you. Because there was a difference in opinion, I assumed it wouldn't have been as clear as that. Will keep that one in my back pocket for the next time I see this player.

And since more sources is always a good thing, It's also referenced in the FAQ under "Rule Clarifications: Measuring Range"

"Players may only measure range and/or use the range ruler to determine whether a ship is inside or outside of a firing arc at the following times:

• When a ship becomes the active ship during the combat phase, the active player can measure range from the active ship to any enemy ships before declaring one as its target."