There's plenty of sites, but I kinda like CSGOGem, its an "upgrade" site where it basically rolls a random number between 0-100. Say you have a 65% chance to 1.5x the value of your skin, if you enable over/under you can say it'll roll under 65%, or over 35%.
So if you do under 65% and you get say a 50% on the roll, then you win. You would have also won with a 50% if you had done over 35%

You can also check your previous "upgrades" and see what numbers you rolled, etc.
Can use these to help predict a bit.

I at one time turned $0.50 skin into an Ak47 Aquamarine Revenged FT from just depositing 3 skins worth $0.50 each. I think the lowest value a skin can be to bet is like $0.40 USD
Pretty decent site imo but you can RIP skins.

And as you can see, I had a Gut knife slaughter just a little over a week ago, and now I have a Karmabit lore. \o/
But yea lol