So I can find musk rats. Have been looking for mink sign. Can't seem to do that. What are some subtle signs I can look for?

I look for mink sign in the late summer,early fall.Especially scats on culverts,rocks in little feeder streams etc.Tracks in mud under railtop culverts,or bridges etc.Also trails in grass at old beaver dams.A lot of these will be made by rats but mink use them too.These will give you an indication/estimate of the overall available trappable population for the upcoming fall season and how much effort if any you will want to expend on setting.
When it comes to actual setting I don't look for sign that much.If I know the mink are available I give them an offer they cant refuse at the classic travel routes.
If you are cold rolling in early winter look for sign in the early snow.In these cases it pays to set on sign.

tracks in mud
droppings on pipes where they go under roads
now is a great time to look especially after some snow. You will learn a lot from their track in the snow.
they are usually where ever there is good habitat if you do not see tracks

Snow tracking works for me.

Right now the rut is on and the mink are moving good, especially the males. You should be able to find mink tracks along the edges of creeks in soft mud or sand. As others have said, also look under bridges and culverts.