timing chain questions

I have a 85 617.952 sedan with 275 K. Absolutely unknown maintenance history.(The last owner seemed to be a maintenance clueless meth head). Great engine.No smoke, no oil consumption,starts well, gobs of power and 27 mpg. Am nervous about the timing chain but don't want to replace it if it has been done. How to decide? By stretch? If it is stretched should I just put in the appropriate woodruf key? Hate to put in a chain if it doesn't need it. How do I tell if its a replacement engine. It's hard for me to believe this is the original motor because of the way it runs.