Pokeadvisor.com is easiest IV checker

This

And if it's something you can evolve regularly, like evee, worth it to wait for 90%+, but the not so easy ones, let's say charmanders, little lower is fine, but I wouldn't waste dust on anything below like 75-80%, specially with Pokeadvisor letting you know what's up.

Move set is also important, evolve the higher iv ones and only if good moves dump dust, otherwise wait, specially if you plan on playing a few months +, something better is bound to come along

So I have cp 600 eevee with 57 iv, this will turn out higher cp than if I evolve my 620 eevee with lower iv?
Not evolving it since not 90+ iv, just wondering.

It's not about what cp it hits when evolved, it's about max cp when powered up, the one with higher iv will be higher cp if leveled to max

to accurately gauge your pokemons IV's you will need to power them up a couple times.
There are lots of variables.

OR just use pokeadvisor....