How do you think Snape evaded being detected by Voldemort all this time?

I think he might have been in constant struggle between the two.

Remember that when Snape began as a death eater, he was desperate and felt as though he had nothing more to do with his life or look forward too. He had been bullied, lost his true love, and had absolutely no friends left. He was hopeless and forlorn and that is exactly what Voldemort saw in him.

Snape for his part played his role extremely well, he had a lot of control over his emotions which helped immensely and always kept on an impassive face. He was also an excellent Occlumens so he was able to shield his mind from Voldemort's prowling. Hence enabling Voldemort to simply rely on Snape and learn to trust him. After all he needed his inside information, as well as the potions Snape regularly brewed for him. Plus when did Snape ever give reason to Voldemort to doubt his allegiance?

Maybe because Voldemort already trusted Snape, so he never felt the need to inspect him?

I'm not so sure, I've never read into that too much.

He seemed to play both ends against the middle. And Voldemort is so egotistical, he misses many things. If you really piece the story together, there are numeous "holes". I guess we just go with it and accept it rather than try to explain every item.