I found a product I would like to sell and improve upon later. Their website says it's patent pending, but they have claimed patent pending for several years. How do I find out if they did apply for a patent?
Patent pending means that either a provisional patent has been filed or a non-provisional has been filed and has not been finally rejected, abandoned, or issued. Provisional patents are only valid for one year and do not get published. Assuming there is no national security restrictions, all non-provisional US patent applications are published after 18 months or sooner if examined/issued before hand [MPEP 1120 R-07.2015].
Searching for the company in a patent search such as Google patents or the USPTO database is simple, but failure to find the company does not mean there is no patent. Just because they offer a patent pending product, does not mean they own the patent. It may be owned by a third party which means you have to do a patent search for the item itself.
If there is no published application or patent after several years then it means that the application was a provisional application and no non-provisional was filed nor was the provisional application converted to a non-provisional; or the company lied.