How to check cap value

How to check trimmer capasitor value. There are no sign of valus printed

Charge the cap through a resistor? After one time constant the voltage will be 0.63 of the applied voltage. My only worry is that the value of the trimmer cap might be so small that your rise time of the charge voltage will be too fast.

Use any capacitance meter that will measure down to pF values, here is one you can build;
https://www.romanblack.com/onesec/CapMeter.htm

A person (not me) is selling low-cost kits for my capmeter design too, if you prefer to buy a kit.

Some series of trimmers are colour coded, and some have house codes on them, as mentioned the best way is to measure it.
You might get some idea as to what circuit its in, ie if its part of an xtal osc then it could be around 35pf.

Romans circuits work, I've never made the cap tester which looks dead simple to build, I've built others with success.