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 charged
Eg for Lewis bases NH3 and (OH)-
Eg for Lewis acid AlCl3 and H+
Here's a helpful page on the topic:
Separate names with a comma.