Another answer by Chris gives better variant.
Navigate in your browser to a list in quesion > on ribbon press "Customize Form" button (under List Tools > List tab > Customize List).
This will open customized before by you Sharepoint List Form in Infopath Designer 2010 which you can save wherever you want.
Though, from your wording of actions (which are usually vice versa), I am afraid that by custom list you call something else like library. To find the most recent Infopath template published to library navigate to a library in browser > on ribbon click Library settings button > Advanced settings > the Document template textbox will show you a relative to your site path, by default:
if you have not customized this location.
Though, if you changed published your Infopath XSN form template many times, then the old Infopath XML forms will be referenced by href in heading of saved in library Infopath forms .XML files... if you have not relinked those files to the most recently published Infopath .XSN forma template.
For example, if URL of my (form) library is:
it is opened on navigation as
should start download dialogue of that template. It can "fail" on 1st attempt, just try once more.
Or, better (since it is not rigid location), in Sharepoint Designer 2010, on ribbon, File > Sites > Open Site (subsite) > expand All Files > NameOfLibrary > Forms > right-click template.xsn (if it was not changed) > Open with > Microsoft Infopath Designer 2010, amongst other options.
It should contain embedded resource files or references to network/sharepoint site path locations.