In the past I was somehow really lazy when it was coming to do really clean models. But since I am involved in the development of Empires and Tribes I learned a lot.
Checking your models is key. But what should you check. Blender gives you a bunch of tools to do so.
Doubles are the first thing I am looking for. Blender gives you the Funktion "Merge by Distance" This is one of the functions I have in my Quickmenu.
If you do gaming models it is very important that you always check the direction of the Normals, I not, Backface-Culling will hit you in the Face
UV-Mapping and Textures
Check always your UV-Mapping and Textures. Not once, you should check it twice or more 😉
Point of Origin
The Point of Origin or saying the root of the model location should always be 0,0,0
Rotation - Scale
Always, Always check the Rotation and Scale values of your model Rotation always to 0,0,0 and Scale to 1,1,1
My preferred export format is fbx. I always limit the Export to Selected Objects and check the Apply Transform option to be sure that I have not to change the orientation afterwards in the Game-Engine, because every action you have to take in the Game-Engine to change, except for the position, your model is something the Game-Engine has to work on. The keyword here is perfomance.
Just my two Cents about how important it is to check on your game models.