We are looking for 3 things:
1) Being able to sit up and hold their head up - they might be a bit wobbly but that's ok
2) Being able to pick something up and put things in their mouths
3) Being able to move food from the front of the mouth to the back of their mouths. If they push it out with their tongue, they aren't ready.
These usually occur around 6 months when their gut, immune system, kidneys and gag reflex are mature enough to cope.
There are lots of other signs people mention like watching you eat, being hungry after a feed, and chewing everything but these are not signs they are ready in themselves.