Shouldn't a law be judged as constitutional or not, before it is passed?

Of course it should. Congress has historically done that, but not since around 1930. Many congressmen would vote no saying "This exceeds the constitutional authority of the government", etc.

Similarly, the president would veto bills he felt were unconstitutional.

Lately, however, this is left entirely to the courts, which is totally wrong and contrary to the oaths of office, and leads to bad laws which are enforced for a while before someone goes to jail for it and an appeal successfully overturns the law.

I am all in favor of a law which would put congressmen in jail if they voted for a law which later was ruled unconstitutional.

The role of the Supreme court is to determine the constitutional status of federal laws passed that are brought into question before them. It is not likely that this will ever change as trying to rule on constitutionality of each legislation passed before it is enacted would bog down the supreme court to just about ineffective. Also remember that the justices appointed to the court have very differing interpretations of what the constitution states, so even though a law may be agreed upon to fall within constitutional guidelines according to the current court, in five years that decision could be reversed.

ADD: Slaps brings up a valid point here. The problem is that congress and the president don't have the authority to rule on the absolute constitutionality of a law. They can as individual members READ the legislation and decided to vote for or against based on their own determination of constitutionality and the implications of the legislation as it applies to Americans as a whole. This practice would be a good one to follow for congressional members and the president, however that would require our elected officials to take initiative in their jobs and actually perform some semblance of work, good luck persuading them in this practice.

Only the courts can authoritatively determine if a law is constitutional, and courts don't give advisory opinions. Also, some laws are constitutional on their face but unconstitutional as applied. So, constitutionality can't always be determined before the law goes into effect.

Article I, U.S. Constitution : Established legislative powers

Article II, U.S. Constitution : Established executive powers

Article III, U.S. Constitution : Established judiciary powers

These three combined are known as the "seperation" of powers. It was formed this way to keep from being a totaltarian state, our founding fathers despised tyranny. One branch of government makes the laws, one enforces the laws, and one interprets the laws. They all check on eachothers powers to make certain no one branch overpowers the other two. Our fathers did not want a tyrannical establishment with full power and control.

It's not Congress or the President's job (nor do they have the authority) to deem anything Unconstitutional...interpretting and defining Law is the job of the Supreme Court.

Laws can't be forwarded to the courts until after they are passed. That's how the balance of power works.