What are best ways to take o2 sat if it is hard to measure with pulse ox? any tips and tricks?

Make sure you're not to cold as this can restrict blood flow to your fingers and give a false low sat as can excessive skin thickness or very dark pigmentation. Keep your finger still and below heart level during measurement. There are expensive pulse oximeters you could use. Otherwise the only other way to measure O2 sat is to draw blood out of an artery..

Do an arterial stick and measure blood gases.