Would you expect to see single mode fibre for short runs? Also easiest way to find out what fibre is currently installed?

Doing some work and for whatever reason we assumed the fibre was MM and turns out it was SM. Thing is when scoped we just assumed it was MM due to the short run involved. Rookie mistake to assume I know.

Would you expect a short <100m run to be MM? Fibre went in about 10 years ago from what we can tell.

Also for the future what sort of tips are there to work out what sort of fibre is installed if it’s legacy links for which no documentation exists?