Blockages of up to 90%, whilst considered 'critical', typically cause little in the way of symptoms.
The gold standard for detecting vessel blockage is angiogram- puncture the femoral or radial artery, feed a catheter to the top of the aorta and squirt some dye and then watch it on X-ray.
This has risks however- risks of rupturing artery walls, causing bleeding and bruising, and can be expensive. So it is not routinely done: in fact it is only indicated if a patient presents with a MI (gold standard is to have the patient in angio lab within 90 minutes of onset of symptoms, which is great because if they are they can generally reverse the blockages with stenting before long term damage occurs), or if a patient has a positive stress test.
CT angiograms are becoming more common but still not commonly used due to decreased sensitivity.
If a patient is believed to have risk factors for artery disease then they may be given a exercise stress test- hook up to an ECG, get on a treadmill and if there are changes consistent with vessel ISCHAEMIA (as opposed to infarct), then an angiogram will usually be scheduled.
For patients too unwell for a exercise stress test nuclear medicine stress tests can be performed.
And finally in patients who have blockages in more than 3 arteries, the gold standard is still (for the time being) coronary artery bypass grafting.
I guess to summarise, the reason we can't detect it earlier is because generally we require invasive means to do so and invasive tests carry risk to the patient that are hard to justify for screening purposes.
Perhaps in the future CT angio may be made more readily available, the scanners keep getting better and better and resolution increases all the time however you always want to minimise a persons radiation dose as for every 3-400 people scanned you can cause one instance of cancer... Something you must always be mindful of particularly in children
