There are plenty of DO meters available these days, and reasonably accurate units have come down in price, to reasonable levels compared to a decade ago.
You can get one for about $80.
But as Deadeye said, whether or not they are nessecary is the real question.
If you are keeping up with water changes, and provide enough surface agition for the type fish you are keeping you should be good.
If there is a power outage during the night they may end up dead anyway.
We had a planned, and expected power outage today on the island where I live.
So as soon as the power went out, in went the ayirstone from my battery operated air pump, and all was well until the power kicked back in.
More than a meter, money might be better invested in one of these.
I take the one below along, as part of my standard kit, every time I collect fish