One or more of these usually show that the other end has no clue about "Free and Open Source" philosophy. or they are afraid to give you the real clue about the philosophy :-) .
1) They "use" GNU/Linux and other FOSS products, but their product itself is closed and proprietary. In other words, it runs over "Free and Open Source Software", but is not "Free and Open Source Software" itself. This also includes plug-ins and extensions for existing "Free and Open Source Software".
2) It keeps consuming and spewing data in an extremely non-standard and non-flexible format.
3) All development is done in-house and other developers are warded off if they come into their "territory", at all times
4) All the source code is available, but you can compile it and use it only if you have proprietary compiler/ toolkit.
5) The product is not a complete software, but is a value addition.
6) They build a custom software for you , they tell you it is open source, because the OS it runs on, any server it runs on, The interpreters it uses , the compiler they chose everything is "Free and Open Source Software". But they are not ready to give you the source of the custom Software they built for you.
7) They don't have a proper "copyright" and "license" section anywhere in the Source, and /or don't give credit to those people from whom they got code snippets, and/or the libraries where due.