In Vayikra 25: 20-22 (below) God says that he will guarantee a bountiful harvest if the Shmita is observed. Unlike other promises of spiritual reward, this one is easily quantifiable with observable measures i.e. quality and quantity of produce.
Can, ought, and how should this guarantee be tested to probe this matter?
20. And if you should say, "What will we eat in the seventh year? We will not sow, and we will not gather in our produce!" כ. וְכִי תֹאמְרוּ מַה נֹּאכַל בַּשָּׁנָה הַשְּׁבִיעִת הֵן לֹא נִזְרָע וְלֹא נֶאֱסֹף אֶת תְּבוּאָתֵנוּ׃
21. [Know then, that] I will command My blessing for you in the sixth year, and it will yield produce for three years. כא. וְצִוִּיתִי אֶת בִּרְכָתִי לָכֶם בַּשָּׁנָה הַשִּׁשִּׁית וְעָשָׂת אֶת הַתְּבוּאָה לִשְׁלשׁ הַשָּׁנִים׃
22. And you will sow in the eighth year, while [still] eating from the old crops until the ninth year; until the arrival of its crop, you will eat the old [crop].