At the moment there is no way to conclusively determine a monster's level other than the XP rewarded from killing it, relative to your player's level according to How do I determine the level and XP reward of mobs in Diablo III?.
However, if you do determine a monster's level and kill it, does it reward the same XP as any other monster the same level? Is the amount of XP affected by type, e.g. ghoul vs carrion bat vs grotesque? Is the amount of XP affected by monster status, e.g. normal monster vs champion vs unique/miniboss vs boss?
Best Answer
Yes, but there are some significant caveats.
There is a very helpful table here which lists the experience scaling for your level compared to a given monsters level. It is also untrue that you can't determine the level of a monster - but I don't think there is a way to determine it in game. I've also been taking measurements for other questions in various parties, and I've found the output to be consistent with the conclusion that monsters of the same level always award the same amount of experience.