Q:

Is there a way to see the machine behind Sans' lab?

I recently found the lab that Sans has. However, I couldn't see what the machine looked like. Is there a way to see it?

A:

No, you cannot see what the machine looks like. 
Sans' Workshop is an unfinished and/or bonus part of the game and doesn't really impact the plot in any way. It has several mysterious objects which fans have theorized reference Gaster (the previous Royal Scientist) in some way. The machine under the cloth is believed to be a Gaster Blaster, the dragon-head-shaped laser guns Sans uses against you int he Genocide route. However, none of these theories have any concrete proof since Gaster is an unfinished character that can only be seen in-game by hacking your save file. 
For more information about Sans' workshop, you can take a look at the wiki page on the subject. 

