It's easy to know when your jam is set. While the mixture is cooking, refrigerate a small plate. Then, near the end of cooking time, put a small amount of the mixture onto the plate (say, half a teaspoonful - I always put the sample on the edge of the plate, the coolest part). It will cool quickly, and if you can push it around with your finger, and it wrinkles, then your jam is set. If it's a bit runny still, cook a little longer. Things can happen quite quickly near the end of cooking time, so do the test every minute or so, making sure the plate stays cold.
Alternatively, and only if you're brave, give the jam a quick stir, lift out the spoon and wait until it cools a bit. Then run your finger along the spoon. The small amount you take off onto your finger will cool very quickly, and you can tell from its consistency whether the jam is set or not. I've been known to burn said finger more than once using this method, so be careful - a wooden spoon will get less hot than a metal spoon! You will notice, too, that the spoon will stay coated much longer when the jam is ready.
If the wretched stuff just WON'T set, add a squeeze or two of lemon juice and cook a little longer. I've never known this to fail, and I've never used commercial pectin.
BTW, most jams require a really full rolling boil when cooking. None of this wimpy simmering thing! You need to watch your jams - things can happen quite quickly. Like burning on the bottom of the saucepan. Use common sense and experience to judge how high your heat is.