HOW DO YOU judge the dramatic and complex changes in the income tax that President Reagan outlined last night? There are two essential tests: whether the president's proposed system will be as fair as the present one, and whether it will raise as much money. Simplification is the purpose of the president's plan, and simplification of this country's endlessly and mindlessly intricate tax code is devoutly to be wished. But any broad attempt at simplification will inevitably collide with various concepts of equity that have been stitched into the code over its long history. And this plan has to maintain the present level of revenue. Mr. Reagan himself set that requirement and, in view of the gigantic budget deficit, it's indispensable.

The president has not provided many details yet. When a president speaks on a subject of such intricacy as this one, the White House usually puts out, hours in advance, extensive technical explanations of the points that are unavoidably compressed or neglected in a televised address. It allows the press and others with a close interest -- lobbyists and trade associations, for example -- to understand more precisely what the administration wants to accomplish. But in this case the White House has refused to distribute any of the usual supporting analysis until later today.

Evidently the president wanted to establish a mood and an impression, unburdened by the kind of questioning that a more complete and explicit description of the legislation might inspire. It's a legitimate tactic on the part of a president who wants a 12-hour jump on the skeptics. But it's also a reason for taxpayers to reserve judgment until the congressional hearings are under way.

Tax simplification deserves strong and sustained public support. But there is quite a lot in Mr. Reagan's plan that doesn't have much to do with making the income tax simpler -- for example, the $960 increase in the personal exemption. Since everybody gets an exemption, it's a tax cut for everybody -- but not equal for everybody. Like any exemption, its value rises with your income. If you're in the 15 percent bracket, it's worth $144 a year to you. If you're in the 35 percent bracket, it's worth $336. Not only does it contribute nothing to simplification, but it doesn't do much for basic fairness either.

The original idea was to begin by abolishing most of the special preferences and shelters. That in turn would make it possible to drop the tax rates without any loss of revenue to the government. But as it now stands, the president's plan seems to be much clearer about the lower rates and bigger exemptions than about the attack on the shelters and preferences. Another tax cut is not what the country needs. Even the most passionate supporters of a simpler tax code will want to hear more -- much more -- before they enlist in Mr. Reagan's crusade.