Instead of carrying my multi meter around I have one of these smart duel USB adapters which has a digital display of the battery state of charge.
It's cheap as chips & works well for £2.92 delivered
https://www.banggood.com/FN05-Dual-U...r_warehouse=CN
At a glance I can see my standing voltage 12.6 when I turn the key on & 14.4 when the engine is running.