How can I distinguish between Lewis acids and bases?

[sidt36]

How do I know which species is a Lewis acid and which is a Lewis base?

 

[Karan Punjabi]

How do I know which species is a Lewis acid and which is a Lewis base?

A species which has a lone pair of electrons is recognized is as Lewis base the species can be either negatively charged or neutral. A Lewis acid is a species which can make bond by accepting lone pair of electrons it can be neutral or positively chargedEg for Lewis bases NH3 and (OH)-
Eg for Lewis acid AlCl3 and H+

 

[Ygggdrasil]

Here's a helpful page on the topic:
http://www.chem.ucla.edu/~harding/tutorials/elec_nuc/elec_nuc.html