A few deliveries before a match may feel like a sensible warm-up, but competition practice can affect fairness. Ask the controlling body and keep practice separate from trial ends.
Do not test your drawn rink
Under World Bowls law 3.3, playing on your drawn rink before the day’s competition or game can disqualify you, with an exception for open tournaments. Domestic regulations can set the detailed local requirements. Confirm the draw before rolling any bowls.
When practice is allowed
Law 4 allows a player or team that has not yet played to practise before meeting someone who has already played that day, but only with controlling-body approval, enough time, and another available rink. The controlling body allocates that rink.
What about a bye?
A player who has already played on the green that day may also practise under law 4 when they have a bye, subject to the same conditions. If two eligible sides practise together, they can agree the format and number of bowls.
Trial ends are separate
Trial ends are part of the match-start procedure, played on the match rink when permitted. They help both sides read pace and line under the same conditions; they are not permission for a private early practice session.
Source: World Bowls Laws of the Sport, laws 3–5. Check your national regulations and event conditions before a competitive game.
The safe routine is simple: ask the organiser where, when and whether you may practise.
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.