I cannot use sf::RenderTextures in that computer
Why not? If hardware acceleration isn't available, SFML will fall back to emulation. sf::RenderTexture should always be available, even on really crappy systems.
when i draw directily to the screen without doble buffer the cpu goes to 50% instead of 3%.
What do you mean by "without double buffering"? SFML will always use OpenGL double buffering. If a double buffering pixel format is not available, context creation will fail and you won't see anything at all. Double buffering isn't something you have to do yourself...
What can i do to detect this in my code and show the correct message for the player? By the moment the message is seen in the console.
All console messages are for debugging purposes only. In release, you would either want to disable the console all together or divert the output somewhere else.
A relatively simple way to tell if the system isn't being nice with OpenGL is to check the version of the context after you created your window. If it is 1.1 or lower then it is pretty safe to say that it is not truly hardware accelerated. You can also include OpenGL.hpp and read out the OpenGL implementation strings if you feel like it.