The Palestine Liberation Organization appears to be on its best behavior - despite allegations by Israeli officials in recent weeks that the PLO is marshaling men and equipment in southern Lebanon.
Monitor conversations with PLO leaders, with United Nations officers patrolling the hills west of Tyre, as well as firsthand verification along much of the border, indicate that:
* PLO strongholds in the Tyre, Dayr Amis (or ''Iron Triangle''), and Beaufort Castle areas have been uncommonly quiet the past six weeks. The two most well-organized PLO brigades near Beaufort Castle have received new Soviet armor and artillery, but not enough to launch an offensive.
* PLO guerrilla and non-PLO attempts to cross into Israel have not been occurring in the past six weeks. Since July 24, 1981, they are down dramatically. In December, Israeli-backed forces commanded by Lebanese Army Maj. Saad Haddad encroached on Irish UN positions near Bayt Yahun, gained control of a strategic hill, but stopped there.
* PLO strategy is to prepare for an Israeli attack, but not to give Israel grounds to justify an attack. PLO officials admit privately that under a concerted Israeli offensive they would be forced to fall back to the coastal orange groves between Tyre and Sidon.
* The extra 1,000 UN soldiers approved last week for deployment into southern Lebanon may go toward bolstering current UN positions and - more importantly - to plugging the dangerous gap in the Beaufort Castle area where Palestinians and Israeli/Haddad forces are in direct opposition.
If the gap can be filled with UN soldiers it will significantly lessen the likelihood of a direct Israel ground invasion. This is what UN officers familiar with the area think is needed, but whether the Israeli-Haddad or Palestinian forces in the gap allow such a deployment is uncertain.
The greatest source of concern until that gap is sealed is that an overly ambitious guerrilla group might attempt to infiltrate into Israel. But for the past month the PLO has taken a low profile.
According to Col. Bert von Tol of the UN peacekeeping force, the guerrillas ''know what we all know from listening to the news: Some action would be seen as a violation of the July 24 cease-fire and Israel will come in.''
And at PLO headquarters in Beirut, spokesman Manmoud Labadi asserts that, at least as of this writing, ''We will continue to abide by the cease-fire. We will also continue resistance - including armed resistance within the occupied territories. But we have made a promise and we will keep it with regard to action across the Lebanese border into Israel.''
Mr. Labadi says the PLO is trying to see that this does not happen. The PLO spokesman also says that the stream of Israeli threats of retaliatory and/or preemptive strikes are part of an Israeli attempt to convince Washington that an attack, breaking the seven-month cease-fire, is necessary:
''Israel is used to puffing up its accusations like a big balloon,'' charges Mr. Labadi, ''in order to give its adversaries a bigger size than they have in reality. They are doing this in order to extract a green light from Washington to attack.''
Israeli patrol boats cruising off southern Lebanon were fired on Feb. 28, but the incident appears to have ended at that. US special envoy Philip C. Habib met with Israeli Prime Minister Menachem Begin in Tel Aviv March 1 in an attempt to insure that such incidents do not cause the cease-fire he helped bring about last summer to break down.
UN officials warn that even though there is the appearance of quiet in the region, the guerrillas can organize quickly and the Israelis can strike without even massing forces along the border. But these same officials say the frequent Israeli warnings of an imminent attack may already have served ''psychologically'' to keep the guerrillas on best behavior, as well as giving the UN the sense of urgency that caused it to vote another 1,000 soldiers for the border area.
''It's certainly less costly to talk about invasion than it is to actually invade,'' an Irish officer says.
''Not long and there will be 7,000 UN soldiers along this border,'' the officer says. ''Then there are 1,300 UN men along the Syrian-Israeli border, the 2,000-plus men in the multinational force in the Sinai. They've a peaceful border with Jordan. They will be buffered completely.''
On this side, if the cease-fire continues to hold, life for the Lebanese villagers and farmers may return to normal. Already the net effect of the seven months of peace is a flow of refugees back into the UN-controlled areas and a house-building boom throughout southern Lebanon.
''It's a tricky assignment for us, but it's worth it to see confidence coming back slowly but surely,'' the officer says.