If you connect the solar panel to the battery
without a solar controller, the solar panel voltage will always be the same as the battery voltage (ignoring voltage drop in the wiring). The same is true if your use a non MPPT
(often called a PWM) controller.
If your open cell solar voltage is 25v you need a MPPT
controller. You can then use a simple voltmeter to measure the solar panel voltage. Some controllers (such as the popular Victron Smartsolar) report the solar panel and battery voltage so no voltmeter is needed. With an MPPT the solar panel voltage and the battery voltage will be different.