Given life and conditions we see today -or- how things could be if designers were actually present and persistently active?
Today, you’re basically left with common descent and modification in stages rather than special creation or release of unrelated species/organisms. If you’re looking for evidence of design, it’s probably best to examine test cases where we can know the most about the initial and final conditions for a putative, design event.
One heavily worked example is the human/ape split. That’s relatively recent and most predictions for where ‘design case’ arguments could be made center around the dissimilarities in genomes between chimps and humans. One claim was that the emergence of humans required a large number of genetic changes that would be hard to account for via known biochemical mechanisms. One idea was the the number of ‘ORFans’ for which no related sequence similarities are known would increase as we compared more of the various primate genomes. (pace Paul Nelson. In fact, the number of unaccounted-for ORFans dropped with better sequence data). One signal that remains persistent is a strong correlation between then time since proposed divergence and differences in sequences. There is no a priori reason why that would be the case if species were specially created, but alas, that’s not the pattern observed.
Front-loading was proposed at one time (also be panspermia proponents), but not much workable research has emerged to date. Plus, front-loading would be largely consonant with much of the evolutionary change we’ve inferred.
With this and many other cases for which we have the most reliable coverage data, it would appear that a designer likely implemented their design in ways that make it hard to differentiate from understood mechanisms we see in cells today.
Old-Earth design is admittedly, harder to differentiate from current scientific theories than young-earth proposals. With YEC, you’ve got proposed physical events like the Noachian deluge and massive, recent, population bottlenecks across many species that should present very strong and unambiguous evidence.
So, given the strong signal of descent over time with modification, one might investigate:
Orthogonal coding in DNA sequences (steganography, independent of a functional role in cells)
Identification of design-capable intelligences present during much of Earth’s history: i.e. relics of occupation, communications, & etc.
…Admittedly, those would provide more direct evidence for an agent of design operating at any particular point in Earth’s history. But confirmation in any of those areas would greatly raise the support for direct, manipulation by a designer.