Didn't guess a runner out...wrong?

[BillM.]

ASA Men's 12'' sp. 1 man ump mechanics
Runner on 1st. No outs.

Batter hits a laser to the pitcher, who catches in in air. Out 1. Runner takes off to second, on contact, no tag. The pitcher slings the ball to the 1st basemen, who isn't on the bag, and swipes at the runner returning to first. I am running towards the 'c' position to get a good angle, but I didn't get in position in time to see the tag. It looks like there was a missed tag, as there was no sound of a tag, nor did the arm of the 1st baseman slow down as if he made some type of contact. The runner was in between me, and the 1st basemen, who was trying the tag from the outfield side of the runner, if that makes sense.

I did not clearly see a tag, so I call the runner safe upon his return to first. They appeal, but I deny the appeal since it was my judgement call.

How should I have handled the situation, and how could I have been in better position to see the tag?