Geekcreit produces mini digital voltmeters for general purposes. Because the price of such a voltmeter is low (less than 2 euros) you can incorporate the voltmeter into each of your do-it-yourself projects.
Range is about 2.5...30 VDC and the voltmeters, to be found on aliexpress or banggood.com, are offered in various colours: red, yellow, blue, green and white.
I used one on my 144 MHz transverter: