1. How is your movement?
Nothing is more frustrating than realizing that you do not have enough movement to close the distance to your foe or to get out of town when things go bad. Rest up before you even check the area where you suspect they are in.


2. Did they see you peek in?
If you believe they saw you, leave them be. Come back ten-fifteen minutes later, after they've gotten into a rhythm that is more concerned with ranking than in looking for pkers. If not, proceed to step 3.


3. My character build
A high damage build is optimal in that you can kill and give the group minimal opportunity to react. If you are a lagging build, attack them when they are fighting, as the mob will interfere in the groupmates' opportunities to aid them. Ideally wait until they have been fighting some so the whole group is weakened in health and mana. If I can use stealth to get near them, I like to attack when they are resting, with one or more of them afk, or to time my attack when the tank engages the mob and group assist has not yet kicked in. If I'm trying to assassinate, then I'm going to weigh factors maximizing my chances of getting a kill against the odds of the character leaving the ranking area.


4. Target Group makeup
If the group is squishy, be aggressive. If the group is tanky and you can use stealth, then wait for when you perceive a weakness. If the group has a ranger or assassin, I definitely want to engage them as they fight.


5. Terrain & Mobs
Going back to movement-make sure you have enough for your goals.

Some areas are not easy to find people in, so charging in could simply give them time to get away. This is particularly true if you do not have pathfinding and they are in areas associated with high movement lag. If you have aggro mobs that will interfere on a charge, then I wouldn't go charging in either, as they will announce your presence.