How do I check normal capacitors(not electrolytics or non polarised electrolytics) in circuit(unpowered) with only a digital mulimeter without desoldering one leg?

 

Easy - unsolder everything else.

 

Unsoldering everything else is much more work than is necessary. If you don't want to unsolder one of the legs, you can just unsolder the other leg!

But seriously, the trouble is that other parts may interact with the cap.

But if you know how the circuit is supposed to behave, sometimes you can check for good conditions, such as a low value of ripple voltage on a filter cap, or a low amount of RF on a bypass cap.

If a cap has opened, you can sometimes do a test of bridging it in the circuit with a good cap to see if the circuit then works properly, or you can test for different DC voltages on the two legs to check whether the cap has shorted.

If you cannot power the circuit, and cannot remove the part, all I could suggest is to study the remaining parts for how they might interact with the capacitor, then use the multimeter check for whether the cap has shorted, if that's possible. I don't know what else you could do on an unpowered circuit with just a multimeter.