Congratulations - sounds like you're off to a great start!
Has your partner done an investment like this before? If so, I would lean into him & see how he decides whether he's making a good/sound investment decision or not.
I would start by conducting some sort of market research: look into the area and find out what makes it good. See if there's competition in the area, what supply & demand looks like, vacancy rates in the submarket, trends, and potential growth.
Request a pro forma statement from the developer. This will show projected income/expenses, expected ROI, and cash flow projections. Review the numbers and don't hesitate to ask questions - pay attention to contingencies, development fees, other costs associated with the process.
Understand the legal/regulatory framework - be sure to understand the requirements involved with the dev process, see if the project requires any special permits, zoning changes, or environmental clearances.
Check the development plans - ask questions about the design, layout, amenitites, and if its appropriate for the target market/audience. Consider other factors such as location, proximity to amenitites, and the quality of the surrounding buildings/infrastructure.
If you're specifically looking for tools to use to punch in projections/numbers, BiggerPockets has a rental property + house flipping calculator and a real estate analysis tool. If you're looking for more, just google it and plenty of results will show.
Investing in RE is no quick and easy process, be sure to take your time, research, analyze and make an informed decision that aligns with your investments goals & risk tolerance.