Doubts about a company should arise when:
1. Shareholder does not get any dividend
2. The registered share holder is not getting any information or report about the company, including the notice of AGM
3.The shares are not traded in any stock exchange or very rarely traded
4.The market price of the share does not change for a long period or is on gradual ,decline.
One can know about the company by searching in the stock exchange web sites or the financial websites which give such services . We can also take up with the Registrar of Companies and stock exchange to see if the company has filed any data and details required as per the stock exchange rules and company Act.
A general suggestion here is that unless one is very experienced and expert in stock trading matters, it is better to keep away from penny stocks. It may be just fluke that one of them may give some good profit sometime as a one time wonder. There can be some penny stock companies who may genuinely grow up due to products change, management change, financial infusion et., but then that will gradually show up in its activity and reports.
When a company gets liquidate shareholder is the last person to get something. He may totally lose his investment generally.