This is not the ultimate answer, but just an extra tip in case you don't have a scale and have no idea what to do:
I have seen my self in that situation more than once. I have an handheld scale, but it's not hard to forget it.
What I often do is to weight the luggage at the airport. I just go to an empty check in desk and put my luggage on the belt. Most times the scales are turned on and have a weight LCD screen for the user. I sometimes even do it if I weighted the luggage at home but I am near the limit. Home scales can have up to half kilo deviations and can be hard to read with precision if they don't have a digital screen.
I know weighting in the airport can be sometimes too late but at least you don't go through the embarrassment and pressure of having extra weight and all queue of people waiting for checking-in while you try to solve the problem (either by paying extra or removing stuff).
In any situation, if you don’t want to pay extra, prepare to leave things behind or to be able to quickly move things between hand luggage and checked-in luggage (sometimes only one of them is extra weight) or to some friend luggage that has less weight. Choose the items you wish to exchange or that you are prepared to leave behind and leave them in a position where they are accessible.
What I usually do is:
- If it's a returning trip and I weighted my suitcase at departure (home or airport) I know how much weight I have available. I just try to put the same content (or less) and estimate the extra weight I want to put in (souvenirs, documents, whatever)
- If I have no clue than it's bad, but I try to pick the heaviest things and put it on the hand luggage. Hand luggage can be weighted but there are several situations that make a bit easier to handle. A. some companies don't have weight restrictions for hand luggage; B. you can still do your check-in and than solve the extra weight problem after leaving the queue. You can than return to ask for the "approved hand luggage" sticker if that applies.
In any situation try not to arrive late to the airport so you have time to handle the situation.