That's easy; when the raccoon steals the first one without a trace (they're very picky). Otherwise, pick one and slice it open; if the seeds are pale white or tan, it isn't ripe yet. If the seeds are brown, then they're ready.

Too bad about the varieties; old-fashioned Delicious and the old Rome Beauty (not red) are much superior in flavor to both your modern sports.

Applenut

I used to live in an apple growing area and every year the county agricultural commissioner would issue the starting date in the local paper. Red delicious was a popular apple with the growers because the government was a big buyer of apples and they said the soldiers would only eat red apples. For their own use I never knew a farmer who preferred Red delicious. Al

To answer your question, lift the apple about 90° and maybe give a little tug. If it comes off easy odds are it's ripe. You know to start checking when good (bug free, not rotten) apples start falling.
Seed color is usually good but you have to know what they do in the variety you're checking, some are brown, some black, some reddish and sometimes the apple tastes right but the seeds didn't quite get there. Plus you've already picked the apple if you're checking the seeds.

If you like RD that's all that matters. Don't give too much credence to other people's opinion on flavor. I like extremely tart, my niece doesn't like even a hint of tart.
I loved my nieces Newtown Pippin which she gave me because they are extremely tart. Someone here told me they improved with age so I aged them. They lost all their tartness and were just an average apple after that.

No way Red Delicious should be ripe yet in your zone. Normally it's not ready till about mid Oct. Might be a couple weeks early this year with the heat and early spring if you're in the northeast.

One problem with those ultra-red sports is that it gets hard to tell ripeness- that's how the big commercial growers manage to sell so many awful, unripe apples.

I use the turning tech. mentioned here. If it breaks cleanly with a nice stem the fruit is ripe. Then you can check flavor and seeds.