There are many ways to measure your bike intensity on race day, and you want to be clear on which measurement you'll use to determine success. Is your goal to go a certain pace or wattage? Do you want to keep your heart rate or cadence at a certain level regardless of pace? All of these will provide vastly different results.
At the very least, you'll likely want a basic bike computer that includes current speed, average speed, cadence and distance. Most cycling computers and triathlon watches also sync with heart rate monitors and power meters, which are becoming less expensive and more readily available.
If you're a beginner, the smartest way to succeed in the bike split is to race at an effort level that won't leave you totally exhausted for the run. Quite honestly, if you're a newbie, the easier you go, the better—at least until you have the experience and the proper tests to clarify your different levels of intensity. If you're an advanced athlete, the gold standard these days is racing within a certain heart rate or power zone based on your goals and the distance of the race.