If President Reagan really means it when he says he wants the authority to cast a line- item veto, there's a simple mechanism in place right now that would let him test the impact of this idea starting tomorrow morning -- Congress or no Congress.

This handy tool, completely within the president's control, is an official tome called the "Weekly Compilation of Presidential Documents." It's a small weekly magazine, about the size of an issue of TV Guide, in which the White House compiles every official word uttered by the president or published by the White House speechwriters in his name. Properly used, it could be a laboratory for a one-year test of the president's pet policy idea.

In his State of the Union message, the president repeated one of the pleas he's been making to unsympathetic legislators almost from the moment he entered the White House: "Give me a line-item veto this year," he said. "Give me the authority to veto waste and I'll take the authority, I'll make the cuts, I'll take the heat."

In plain English, Reagan is asking for the power to kill certain specific proposals, or "line items," in a big spending bill without being forced to veto the whole piece of legislation.

The governors of 43 states have this power now. But the president has no authority to pick and choose among line items: he can either veto an entire bill, including the good stuff, or else swallow the whole enchilada, with all its costly filling.

For all Reagan's persuasive powers, line- item veto legislation is probably a nonstarter this year. There are political obstacles: some Democrats don't want Reagan to get the credit if the idea turns out to work. The more important hangup -- the one thing more important than politics in official Washington -- is power. Many members of Congress (in both parties) fear that giving a president this new weapon would shift power over spending too sharply toward the White House end of town.

So the president's chance of getting a line- item veto bill through Congress is just about zilch. But if he really wants to try the line- item veto concept -- if he really wants to "take the heat" -- Reagan need not wait for Congress. He already has the perfect apparatus to test this policy idea: the "Weekly Compilation of Presidential Documents."

A regular feature of this prolix little journal is presidential comment on the legislation signed (or vetoed) during the past week. If Reagan really believes in the line-item veto, he should immediately start listing in these official comments all the line items he would have crunched in each bill if only he had the power.

These pronouncements wouldn't really amount to a veto. But they would force the president to show us how his line-item veto would work. They might reveal the hiding places of those thre elusive Washington gremlins: waste, fraud and abuse.

Every few weeks somebody in Congress, the media or academia could add up all the "vetos" and see how much would have been saved and what programs the nation would have done without. Twice a year or so Reagan could send Congress a list of all the wasteful line items and ask that they be de-authorized.

It could be that Ronald Reagan sees this issue merely as a hammer to use in pounding Congress; if so, he can go on giving speeches lamenting the failure to pass such a bill. But if the president honestly wants to test the line- item veto, he ought to use the "Weekly Compilation" and put his mouth where the money is.

The writer is a member of The Post's national staff.