Pigs are no different in terms of how to tell when to slaughter as any other animal, agree when first starting out probably best to go on weight, but feel is very important too. Should be able too feel ribs as little point/ridges when pressing lightly, but not see them or feel them individually. If you can't feel any bone anywhere then they are fat, and best sent off very quick.
A bacon pig is anything over 80kgs, though in general most people i know send them off nearer 100-110kgs l/w in age around 8 months, but some pigs can be as young as 6months at that weight (80kg). Your piggies should be around bacon weight by now. Do you have weigh scales you can use for them?