DD has been sitting up competently since just over 4 months (and holding her head steady since a couple of weeks old).
She puts everything in her mouth. Nothing misses.
Unless I'm mistaken, the last sign of readiness is swallowing food instead of spitting it out, but I won't know that without giving her some broccoli, or whatever.
She's just over 5 months and grabbing at any and all food we have. It's it time, or just too early?