What is the best way of telling whether to add to the
designated cutting size in a quilt pattern if it does not
specifically mention add 1/4 inch seam allowance?

I am making a set of placemats, I cut one and it just
does not look right, but the pattern does not say add
the 1/4 inch seam allowance.

Generally, the pattern includes the seam allowance.

As for your placemats...you can make them any size that you want. If you think they should be larger, then cut your fabric larger.

Good to know.

usually if the pattern is not written with cutting measurements it will tell you to add seam allowances-
i think most patterns are written with cutting instructions-not finished size instructions.
the one that's not is the appliqued one- that the designer uses raw-edge fusable- if you wanted to needle turn you would need to add your turn under allowance.

Sometimes you can just make a sample out of scraps & see how it turns out.

Your pattern should have the finished size somewhere on it.