Yes, they stay attached to the pod until you remove them.

Open a pod, strip out a few peas and taste them.

Best is when they are grown, but not yet getting starchy.

I think a good way to tell is as soon as they fill out the pod ....

Depends on what you mean by ready, so you'll have to learn by trial and error. Fully mature pods are round and hard, younger ones more oval shaped and give a little when squeezed. Also, the pods are slightly translucent, so you can judge the size of the seeds by stooping down and holding them to the light.

It also depends on what you want to do with them, in terms of how "ready" you want them. If I have fresh peas around, I eat them raw. I don't dare cook them or freeze them - I just can't even imagine that because they're SO good fresh out of the pods. That being the case, I don't like them when they've completely filled out the pod because that means the peas inside are at the stage just before they get starchy, strong tasting, and dry, however, if you're making a soup or something out of them, that's kind of perfect.

If you want them good for fresh eating, or even light cooking, pick them when the pods are fully fattened up, but before they're hard to the touch. The pod should give and almost feel like it's going to crack under the pressure of your fingers when you give it a squeeze without being able to be pressed so far in that the inside walls are almost touching - that would mean the peas aren't big enough yet.

To get a feel for what stage your peas suit your needs best, keep feeling them, and plucking the ones you think may be ready. You'll get a knack for picking out the best ones.

The proper stage can be hard to detect visually, since the pods of different varieties have varying amounts of space surrounding the peas. Most pods become fairly cylindrical when the peas have reached full size. The best way to know when to pick them is, as 3killerbs mentioned, to open a pod or two every few days & taste them. As soon as they are large & sweet, pick them promptly, as the quality will deteriorate within days... especially in hot weather.

After the first few times of determining when the peas are ready by taste, notice their size. I know when mine are ready when the sun shining through the pod reveals their size. It works for me since a very moist pod (sometimes happens after rain) can appear to have bigger peas than they actually are.

Peas are ready to eat when the pod looks full without being overly full--no space for anything more and bumpy. These ones are old peas and don't taste very good. And yes the sun shining through a pod shows yoiu how big they are.

Peas are sweet and tender when they are young, and hard and tasteless when they are old, so even if you get the odd one not quite full they will be tasty