Natives are provided to query player levels/xp, as well as two forwards: OnGainXP and OnPlayerLevelUp. The OnGainXP forward is the most useful, allowing other plugins to get information on xp gained, jumpshots, noscopes, etc.
For instance, you could use it to display a message everytime someone gets a noscope kill:
public void OnGainXP(int client, int victim, int xp, int damage, bool killshot, bool headshot, bool noscope, bool jumpshot, bool grenade_hit) {
// if noscope and killshot are both true then do something here
}
The OnPlayerLevelUp forward could, for example, display a notification when a user levels up:
public void OnPlayerLevelUp(int client, int oldLevel, int newLevel) {
// client went from oldLevel to newLevel, display notification here
}