hi,
i had the same questions when my ds was 3mo. he's now 5mo and we moved him to 4hr easy at about 3.5mo. i think the signs are short naps, meaning he can afford a longer awake time for longer naps. for me i just waited to see if he could hold out not eating for 4hrs.
i think awake time gradually moves up to 2hrs by 4mo. but of course it depends on your child. if you do a search on how to move to 4hr easy there are tons of great posts!