The United States and Soviet Union have agreed in principle to a meeting on verification of limits on nuclear test explosions, a major sticking point in arms control discussions, Reagan Administration officials said. The meeting has not been scheduled but is expected to occur sometime this summer, the officials said. No location was announced. It would be the first such session since negotiations that led to the unratified 1974 Threshold Test Ban Treaty. Officials said the purpose of the talks will not be to negotiate but to exchange information on the complicated and sensitive topic.