I don't think any search for life on other planets begins with looking for and sequencing DNA so the time between getting a false positive and disconfirming it could be a decade or more and be extremely costly to send unneeded equipment to Mars or Europa or wherever.
That's a very good point, I didn't think of this lag. Now considering we just did this thought experiment, maybe it wouldn't be a bad idea for any mission to Mars in search of life to have some cheap way of identifying Earth-based microbes, by, for example, coupling a cheap microscope with some machine learning. It wouldn't replace DNA sequencing, but it could be 95% there. Oh, and by the way, assuming we do find some extraterrestrial life form, wouldn't we want to find the way it encodes information as soon as possible, rather than wait for a decade or more for a follow up mission?
An extremely expensive although effective approach would be to "seed" the spacecraft with isotopically enriched and documented solvents and food sources such that all the oxygen or carbon or both on the spacecraft, if any contamination DOES exist, would be a peculiar and documented isotope ratio that is different than Mars ratios.
I think we know the natural ratios for Mars in bulk material, so anything immediately found to be growing that matched the documented "salted" isotope ratios obviously was a spacecraft contaminant whereas anything mars ratio was local. Of course bacteria can reproduce fast, so Earth bacteria eating Mars material would rapidly revert to Mars isotope ratios. Still for the first hour on the ground this would likely be usable for life detection experiments. Although it would probably be unimaginably expensive.
Are there machines that can sift through a sample of dirt, detect cells and isolate them for library prep and sequencing? Or are you thinking of human missions to Mars?
It seems likely that there will be such machines within a few years. There is nothing in this procedure which can't be automated using existing technology.
