Monster wise?
Click on them, if their name is dark blue, they're very easy (and very easy on the exp).
If it's like blue, they're rather easy (same with the exp)
Green is slightly lower than your level (with good exp)
White is just around your level (better exp)
Yellow is slightly higher than you and a bit tough (and lower on good exp as their's a stupid penalty)
Light red is a good bit higher than your level and very scary
Blood red means stay away.
If you mean other peoplewise, look at their armor, determine their grade and you can pretty much get a good idea what level range they are A grade armor aside of course (as it's level 61-75 and since S grade is so expensive they could be as high as 80)