Q:

Is it possible to check if element 'calling' a mixin is of a specific type using LESS?

To clarify, I have a mixin:
.mymixin()
{
    &.myClass
    {
    }
}

In this mixin you can see I'm checking if the element is of class myClass. I'd like to do the same in principle, but rather than check if it has a class, check if it is a li.
This does not compile:
.mymixin()
{
    &li
    {
    }
}

A:

Yes, this is possible. All you have to do is swap places of the parent selector & and the element selector:
.mymixin()
{
    li&
    {
    }
}

See the docs for more information.

