Hi
Since you intend to join as an investor in an existing private limited company (The word partner is associated with partnership firms, where as in private limited company ownership is restricted to the extent of share holding of directors), you should exercise absolute care and diligence on the following
1) Check with Registrar of companies (www.mca.gov.in)
a) whether the company has been filing their annual balance sheets,
b) Whether the company has any mortgages, loans that are recorded in registrar of companies
c) What is the present share holding pattern and who are the major share holders.
2) Also check with your auditor
a) to ascertain the actual financial status of private limited company (assets, liabilities, cash flow, unsecured creditors, unsecured debtors) by analysing the balance sheet of the company for last 3/5 years,
3) Also since you will joining the company as an investor , you need to be aware of the new share holding ratio of the promoters, subsequent to your investment. Unless you are aware of the new share holding ratio and your share holding thereof , you will not be able to be aware of whether you are a minority share holder or a majority share holder(your auditor/lawyer, can help you with these computations)
4) Also check with the existing directors of the company on whether you will be appointed as a board of director( better to have a board seat, if your investment is more than 10% of paid up capital of the company)
5)Also please check the exit terms- Assuming you wish to take back your money, then how will the company/ present promoters permit you to with draw the money (whether present promoters will buy out your shares or will opt for an IPO etc).
6) In general, whenever a new investor proposes investment in a private limited company, parties enter in to an agreement called "Term Sheet" where parties outline all the details of investment, ownership control, day to day management, bank account operations, exit's etc. So ideally, you should invest only if all of the terms as mentioned above are agreed upon between the parties (term sheet) and parties affix their signature thereof.
7) Finally-Please check whether your investment in the company is going to be invested for expansion of business of company or whether you are paying money for exit of any of the present promoters/director's. In an ideal scenario, your investment should be used for expansion of business of company rather than paying a promoter ( probably a premium) who wants to exit the company per se.
Hope this information is useful.