The SlimLine is a slim, inexpensive Montblanc range from the 1980s. Until 1991 Montblanc made the whole spectrum, from the cheap school pen to the expensive Meisterstück, and the SlimLine sat at the lower end of that programme. Only afterwards did the company concentrate on the high-priced range.
That is precisely why push-button ballpoints from this range are virtually never faked: the resale value is far too low for counterfeiters, and the effort does not pay. What gets copied is above all the Meisterstück and the limited editions. A SlimLine in good condition is therefore very probably genuine, even without elaborate checking. The same argument, though, means the market value is slight: a well-preserved example fetches little on a sale, so as a rule the sensible thing is simply to use the ballpoint yourself (as at 2017).