Hi Amit and welcome:
I assume you are looking at purchasing a used Canon camera, but not sure what you mean by 'original'.
The first thing is to consider the trust level of who is selling it. You want to deal with companies that describe and rate their items, and offer a warranty. If buying from the web, be careful of Facebook Marketplace - there are lots of scams. A good guide is it it looks like an amazing bargain, be cautious.
It would be helpful to know the models of both body and lenses ( be specific on that last as there are multiple models). Generally however, there are things to look for. It's a good sign if the owner has kept the box, packing and accessories, including the manuals. Unfortunately, that is fairly rare, but shows good ownership practice.
A common one is to get the shutter count, but this can be challenging - usually it required a check by Canon to get a certified count, and the camera may have been used for a lot of video, which does not show up as shutter actuations. It is also good to ask if the owner is the original one, or if they have purchased it second-hand, or if it was a rental in its history. If it is supposed to be quite new, is it still under warranty?
However, as important is cosmetic condition. A lot of shine on the grip and around the main controls indicates a lot of use. Look for scratches, missing port covers and that all the controls work as indicated - in that respect you should do your homework on the actual model by downloading and reviewing the manual.
As far as lenses go, make sure they focus and zoom rings work smoothly and for their full range. Make sure any switches work and, look at the elements to make sure they are not scarred and through the length of the lens to check for fungus, which is pretty fatal. Some dust is not really an issue. Of course, try taking some photos and video with the camera. Bring your own cards and ask that the batteries be fully charged before you inspect. It is a cunning trick to say the batteries are flat and thus the function cannot be evaluated.
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
