Can I know my blood sugar level with iPhone?

Take a blood test with your doctor. iPhone, nor Apple Watch measure blood sugar.

On TV, they keep advertising a glucose monitor called Libre, which you wear on your arm and use a companion app to measure your blood sugar. You'd need a prescription from your doctor for that.

You can also get non-wearable blood glucose monitors such as the Contour 1. I use it and I highly recommend it. You don't need a prescription and you can get them at just about any drug store.

It has an app that you can sync your data to your phone as well and keep track of your levels.

https://www.contournextone.com/