I had a crash at approx. 5 mph; the front wheel got caught in a crack in the road, then rapidly turned to the left and I went down. I got up and rode on. The front wheel is slightly bent but spins without touching brake pads, nothing else seems wrong - until I sit up and take my hands off the bars. Unstable, whereas before the crash it was rock steady. I'm guessing this is a fork issue. The bike is 531 frame and fork in a large (62cm) frame from 1981.

So the question is - how can I check fork alignment? Can it be done without disassembly? or should I just take this to an experienced shop or a frame builder?

It's easier and more accurate to do it after disassembly, otherwise it's much down to your skills in eyeballing.
I'd start with flipping the bike upside down, standing behind it and looking closely at how the wheels align.
If you have a patient friend, ask him/her to help you hold a piece of string tight along the wheels, checking that both front and rear sections of both rims are evenly distanced from the string.

Next thing is to pop the front wheel out, and run a string in a triangle through the dropouts and across the top of the steerer tube. Then measure the distance between the string and the crown of the fork on both sides. That too should be equal.

One tricky thing is if you've hit something squarely head on and managed to bend the fork backwards evenly.
That can change the handling too, but won't come up with these string tests.
Best easy option is to look at the bike from the side, and try to create a reference line aligned with the steerer tube.
Thing is, fork legs usually either sit at an angle from the crown, or have funny curves to them. So unless you have a firm idea of how it looked before, determining the amount of change - if any - can be tricky.

When I get a frame that will not ride w/o hands and it is not the usual culprits like headsets, ect. I have found that hanging the bike upside down by string that is run through the rim and then hung directly above the rim (usually do this on a garage door or beam in the garage) that the degree of problem becomes apparent. The front fork will not line up with the rear wheel and when straightened will return to the same spot. A good frame builder will have an alignment table and find the problem. I had a fork and frame straightened (front wheel hung about 30 degrees) and the rear dropouts opened up to 130 last year for $80 so it's not a big deal.

Thanks guys for the input. I'll try your suggestions. I know something is just a little bit off (besides myself).