Sooo... guesstimate...by finding a power that is similar in function, and attached to a monster that is similar in level and role, then pour through the MM's (since we all know there were design flaws in 1... it only makes sense to compare powers in both books.) to first make sure the power you have selected to compare it to is balanced to most powers that are similar, if not select a new power to compare... When this is finally finished then you can actually compare the power you created to a balanced one you have now located...uhm ok.
Making up powers completely on the spot, in 4E,
for me, takes only a few moments and consistently yields absolutely more balanced results than strictly following every formula for monster advancement in 3.5. This is because the numbers largely stay balanced, and it is typically pretty obvious where something falls in the power level: an area-effect at-will stun (save ends) is at the top end, while a power that reduces someone's movement by 1 is not that big a deal.
For the times when you really are unsure, it honestly doesn't take that long to flip open the MM level index, find something similar of the right level, and make a quick comparison. And once you've done that a time or two, you often don't need to for future powers.
Now, I'm not going to claim that such free-form design is going to be the easiest approach for everyone. But I think you are vastly overestimating how much time it takes to check the balance of a power, as well as underestimating how forgiving the system is when adding conditions to powers. And at least for me, I can go crazy adjusting a monster in 4E without ending up with the sort of unbalanced nonsense I could get by advancing 3.5 monsters strictly by the book (either by hit dice, templates or class levels).
Now, was every monster creation in 3.5 flawed? Absolutely not. And for many people, those guidelines remained a solid anchor for their own design. But again speaking from my own experience, 3rd Edition very much felt like it discouraged 'tinkering', while 4E feels like it encourages it, and both makes it quick and easy.
You don't need to agree with this, you are perfectly free to prefer another system, but I don't think it fair to imply that it takes forever to create a new power and quickly compare it to existing ones of that level.