Where to look for logic level of a circuit in the datasheet?
There are usually 4 values you need to find: 


\$V_{IL}\$ and \$V_{IH}\$


These are the input logic levels.  They are often listed as a proportion of the supply voltage, such as \$V_{IH} = 0.6 V_{DD}\$, which is 0.6 times the supply voltage (so for 5V that would be 3V).  The low voltage must be below \$V_{IL}\$ and the high above \$V_{IH}\$.


\$V_{OL}\$ and \$V_{OH}\$


These are the output voltage levels.  They are listed as maximum and minimum values respectively.  Again, they may be listed as a proportion of the supply voltage.