I have planted an apple tree (Red devil) this year , and it have some fruits (10-15). When i check the internet it says its Harvest period is first half of sep. When i tried one which was almost red color , doesn't seemed ripen enough.But some day i can see few fell down on ground (Not sure how) which is having slugs.. My main worry is how to find my the best time to collect it from tree itself.
If you manage to catch a windfall apple before the slugs, then try that one (maybe cutting off a bruised or only slightly slug-munched side, and eating the other). If it's tasty, others with a similar colour will likely be ripe too.
If you can't get lucky with a windfall, give a few ripe looking apples a very gentle twisting tug, if they come away easily they are more likely to be ripe, try one and see if it's nice. If they resist the twist and seem stubbornly attached, leave them be for another few days or weeks. They might still be ripe, but generally the riper apples will be less firmly stuck on.
This is easier on a big tree with more apples, simply because it's less of a loss to pick one apple under-ripe to check. On my tree I just look for the nicest looking one and pick it, if it's nasty I toss it in the compost and try again in a couple of days.
Finally, as time goes by you'll learn roughly when they are best based on the experience of the previous year, it will get easier.