Parshat Bo #2:

How did Moshe know the next plague would be locusts?

Hashem tells Moshe to speak to Pharoah because He hardened Pharoah's heart: Shemot 10:1-2:
וַיֹּאמֶר ה אֶל-מֹשֶׁה, בֹּא אֶל-פַּרְעֹה: כִּי-אֲנִי הִכְבַּדְתִּי אֶת-לִבּוֹ, וְאֶת-לֵב עֲבָדָיו, לְמַעַן שִׁתִי אֹתֹתַי אֵלֶּה, בְּקִרְבּוֹ.
וּלְמַעַן תְּסַפֵּר בְּאָזְנֵי בִנְךָ וּבֶן-בִּנְךָ, אֵת אֲשֶׁר הִתְעַלַּלְתִּי בְּמִצְרַיִם, וְאֶת-אֹתֹתַי, אֲשֶׁר-שַׂמְתִּי בָם; וִידַעְתֶּם, כִּי-אֲנִי ה.
"And the LORD said unto Moses: 'Go in unto Pharaoh; for I have hardened his heart, and the heart of his servants, that I might show these My signs in the midst of them;
and that thou mayest tell in the ears of thy son, and of thy son's son, what I have wrought upon Egypt, and My signs which I have done among them; that ye may know that I am the LORD.'"

But Hashem does not tell Moshe what the plague will be. Yet, Moshe tells Pharoah that the plague will be locusts: Shmot 10:3-4:

וַיָּבֹא מֹשֶׁה וְאַהֲרֹן, אֶל-פַּרְעֹה, וַיֹּאמְרוּ אֵלָיו כֹּה-אָמַר ה אֱלֹקֵי הָעִבְרִים, עַד-מָתַי מֵאַנְתָּ לֵעָנֹת מִפָּנָי; שַׁלַּח עַמִּי, וְיַעַבְדֻנִי.
כִּי אִם-מָאֵן אַתָּה, לְשַׁלֵּחַ אֶת-עַמִּי--הִנְנִי מֵבִיא מָחָר אַרְבֶּה, בִּגְבֻלֶךָ.
"And Moses and Aaron went in unto Pharaoh, and said unto him: 'Thus saith the LORD, the God of the Hebrews: How long wilt thou refuse to humble thyself before Me? let My people go, that they may serve Me.
Else, if thou refuse to let My people go, behold, to-morrow will I bring locusts into thy border;"

Daat Zekenim MiBaalei HaTosafot is troubled by this issue. He suggests that the roshei tevot of the plagues were engraved on Moshe's staff, so he knew what the next plague would be. {I would have a a problem with that, since how did Moshe then know what the Alef stood for.} He then suggests it is because Hashem told Moshe וּלְמַעַן תְּסַפֵּר. And he knew that locusts is something you tell people about, as we see in Yoel 1:3:

עָלֶיהָ, לִבְנֵיכֶם סַפֵּרוּ; וּבְנֵיכֶם, לִבְנֵיהֶם, וּבְנֵיהֶם, לְדוֹר אַחֵר.
יֶתֶר הַגָּזָם אָכַל הָאַרְבֶּה, וְיֶתֶר הָאַרְבֶּה אָכַל הַיָּלֶק; וְיֶתֶר הַיֶּלֶק, אָכַל הֶחָסִיל.
"Tell ye your children of it, and let your children tell their children, and their children another generation.
That which the palmer-worm hath left hath the locust eaten; and that which the locust hath left hath the canker-worm eaten; and that which the canker-worm hath left hath the caterpiller eaten."

I would add that our own local pasuk doesn't only have תְּסַפֵּר but even has בְּאָזְנֵי בִנְךָ וּבֶן-בִּנְךָ which then parallels the pasuk in Yoel even more.

Also, one might say this is entirely a non-issue. Often the Biblical account leaves out details which need not be told, even though they happened, and will bring up the fact that it was told later or else leave it implicit, for the reader to surmise. The best prooftext I can come up with is from Yonah. In Yonah 1:10 the sailors are yell at Yonah because they are upset with the consequences of his actions, yet the Navi does not mention earlier that he told them that he had fled, so how could they know. The pasuk answers:

וַיִּירְאוּ הָאֲנָשִׁים יִרְאָה גְדוֹלָה, וַיֹּאמְרוּ אֵלָיו מַה-זֹּאת עָשִׂיתָ: כִּי-יָדְעוּ הָאֲנָשִׁים, כִּי-מִלִּפְנֵי יְהוָה הוּא בֹרֵחַ--כִּי הִגִּיד, לָהֶם.
"Then were the men exceedingly afraid, and said unto him: 'What is this that thou hast done?' For the men knew that he fled from the presence of the LORD, because he had told them."