Newspapers in Beirut are printing photographs of the new Lebanese Army on maneuvers, firing weapons and operating armored vehicles. But down here in South Lebanon, where the reconstructed army is facing its first real test, there appears to be a wide gap between its capability and its mission - restoring the authority of the central government over the battered region along the Israeli border.

At regional headquarters, the new army looks pretty much like the old one. A few dozen lethargic men are chatting, smoking, drinking coffee, shuffling a few papers. Some wear baseball caps others berets. Some have boots, others wear moccasins with raised heels. Some of their shirts unbuttoned to the navel, expose hairy chests, others reveal red or green undershirts. They have only rifles and pistols, and access to their barracks is controlled by the Syrian army.

No army can be judged by the bored desk-bound troops of the rear echelon, but even the field soldiers of the new army are not prepared to fire any shots in anger.

The army is to be deployed in the south under the terms of a Sept. 26 cease-fire that ended 12 days of fighting in which Israeli forces and Christian Lebanese soldiers attacked Palestinian guerrillas and their Moslem Lebanese allies. The Lebanese army, wholly incapable of intervening, sat it out in another part of the country, and the government concedes it can move into the south only on the sufferance of the warring factions there.

"They don't expect to have to fight, and if they do the whole thing could fall apart," a Western military expert said.

A few days ago the government announced that officers of the new national army had taken "symbolic" control of three major military compounds in the south as a prelude to the deployment of troops into the area. But at the most important of the three, the barracks at the inland town of Nabatiyeh, the army of Lebanon is nowhere to be seen, except for four soldiers who have pitched a tent at a roadside check-point on the outskirts of town.

Nabatiyeh itself is patrolled by Palestinian guerrillas, and the barracks are occupied by a handful of renegade troops of the Lebanese Arab Army, an all-Moslem faction that broke away from the national armed forces during the civil war that ended almost a year ago.

"I read that in the papers, about those officers coming here," said Lebanese Arab Army Capt. Amin Kassem. "They were here as our guests. I am in command here."

The attempt to send the army into the south is seen by all factions in this deeply divided country as a crucial indicator of whether Lebanon can again begin to govern itself after 18 months of civil war and a year in which the only real authority in the country has been the Syrian army.

Lebanon's army, never very strong, disintegrated during the civil war. Some of its approximately 18,000 members took their weapons and joined one or another of the guerrilla groups. Others simply went home, though all continued to be paid.

Now a new commander, Brig, Gen. Victor Khoury, a respected tank officer, has assembled a force of about 1,100 men in garrison in the Bekaa Valley of eastern Lebanon, waiting for the order to go south.

They may have a long wait as political wrangling among Israel, Syria, the Palestine Liberation Organization, the United States and the Lebanese government delays the withdrawal of the guerrilla forces in the south that must precede the deployment of Khoury's troops.

All parties agree that the army has neither the strength of arms nor the political cohesion to fight its way in and impose order, and must wait until it is invited to take over. But military analysts in Beirut say that may be advantageous to the army, giving it more precious time to rebuild itself before it is called upon to do anything.

Military sources say the force Khoury will eventually send to the south will consist of three battalions, one all-Christian, two Christian and Moslem mixed. They have been living and training together for many months, these sources say, apparently in harmony.

Khoury, a Christian, has named Col. Rafiq Zain, a Moslem, as regional commander for the south and Lt. Col. Adib Saad, a Maronite Christian, as commander of the ground forces.

The new army units are equipped mostly with light weapons, a few small tanks and some small artillery pieces that were in the arsenal before the war, military sources said. The United States has agreed to provide some new equipment but it has not yet been ordered, these sources said.

The armed forces also have a few small combat jets that stayed under the control of non-political officers who kept them out of the civil war. Two of them flew low over Beirut the other day in a modest show of force.

Khoury is said to have about 3,000 troops under his authority. His objective is to rebuild the armed forces to their authorized prewar strength of 17,000, or slightly higher, partly through reorganizing and regrouping troops who were already in the service, partly through reintegrating breakaway factions, and partly through recruiting.

Lebanese military officers say they were pleasantly surprised at the response to a recruiting campaign in which leaflets were dropped from helicopters, and squads of shaven-headed recruits, still in their civilian clothes, can be seen marching off to their new units under the eyes of whistle-blowing sergeants near the defense ministry outside Beirut.

Military sources said the recruits will replace prewar soldiers who are so closely identified with a religious or political faction that they would be troublesome in a unified national army.

The attempt to define who is welcome back in the army and who is not is a difficult issue as Moslem troops are refusing to serve alongside Christians who cooperated with Israel during the fighting in the south.