when he can stay dry for a couple of hours at a time, when he shows interest, when he knows the feelings of needing to urinate or deficate (not just after he's done it), he can pull pants up and down by himself (that was a hard one for us!), when there is not a change about to take place (such as a new sibling). Those are just a few indicators that I remember. A big one is knowing the feeling BEFORE they go, not just identifying it after the fact. THey have to be both physically ready and emotionally ready. The latter often comes after the former. Boys tend to be less physically and emotionally ready than girls.
20 months is still very young for potty learning. Many many many children do not potty learn until the age of 3. Experts seem to agree that this is completely normal.
If he's showing you some of the signs, start putting him on the potty. IF he's not, though, don't push it. Pushing causes frustration for you and for him and can cause lots of setbacks. Everyone I know that tried early usually had lots of setbacks. Everyone I know that waited until the child showed that they were ready (not just interested in the potty, but had several of the "signs") seemed to potty learn quickly and without setbacks.
Good luck!