Impossible to be definite about when they're ready, it varies according to environmental conditions as much as variety of pear. However, there is one thing you do need to know - European pears should be picked when they are mature, but not when they appear 'ripe'. They ripen from the inside out, so what looks like an unripe pear is ripening, and by the time its 'ripe' on the outside, it might well be mushy and starting to rot in the middle. So deciding when they're mature enough to pick, but not yet ripe, is the difficult part. The link below might help you choose the right time:-
If you have an Asian pear variety, they will ripen on the tree quite happily and not rot on the inside before they do. As you don't know what you've got there, you'll probably have to experiment this year, maybe picking some which are mature, and leaving others to see how they do.If they ripen nicely on the tree, and are not rotten inside, likely you have an Asian variety - and you'll know for next year.