A close head can tempt players to reach for a tape while bowls are still to come. In a competitive end, that timing matters as much as the measurement itself.
The short answer
Under World Bowls law 23.3, measuring is not allowed before the process of deciding the number of shots starts. That process begins only when the last required bowl has come to rest, subject to a requested 30-second period under law 23.1.
What is the penalty?
If a player measures too early, their side loses the right to deliver any bowls it still has in that end. The other side may play its remaining bowls. This is a real match consequence, so never treat an early measure as a harmless preview.
What can you do instead?
Look from a safe angle, agree which bowl appears nearest, and choose the next shot from that estimate. Keep hands and equipment away from the head. A visual check is useful for tactics; it is not an official measurement.
When the end is complete
Wait for the final required bowl to stop and any properly requested 30 seconds to pass. Then measure between the nearest points of bowl and jack, with the opponents agreeing shots before moving bowls.
Source: World Bowls Laws of the Sport, laws 22–23. Check your national regulations and event conditions before a competitive game.
Practical rule: leave the tape in the bag until the end is ready to be scored.
For a shared view of close heads, try WGTS for iPhone or WGTS for Android. Use the official measuring procedure whenever your competition requires it.