I have some items that gives me +X experience points for each kill, or +X%, but how does this compare to the XP obtained by killing those monsters? I can't see how many points I get when I kill them.
In general, from about Normal difficulty Act 2 onwards, a +10% or higher bonus (from putting a Ruby into a socketed helmet) is going to beat the pants off of any +XP bonuses you can find.
The +XP bonus enchant tends to max out around +15 XP, which would mean it would beat a +10% XP enchant if monsters were worth less than 150 XP per kill. About midway through Act 2, I sampled some of the monsters XP values as I was killing them, by monitoring my current XP to level before and after encounters. Routinely, the average per-kill was more than 200 XP. As you progress through the game, the amount of XP per-kill will increase, and easily dwarf the small bonuses you can get from +XP gear.
This doesn't mean that you should not use +XP enchants on other equipment - but especially later in the game, it becomes less and less worth the investment. Almost always, if you have a choice between a socketed helm with a good quality ruby, or a non-socketed helm with a +XP enchant, take the helm with the ruby.
Bear in mind that the XP you gain from monsters also depends on your level versus the level of the monsters, and information about monster levels is not readily available in-game. Thus, the only real way you can say how much the monsters you are fighting are contributing to your XP is to hover over your XP bar before and after killing them, and then do the math based on these values.