Is the publisher promising a quick turnaround? A quick turnaround time may mean that the publisher is not thoroughly editing the manuscript or rigorously following the peer-review process.
Is the publisher transparent about publication fees? There are some questionable journals who will ask for a publication fee after you have submitted your manuscript.
Are your colleagues familiar with the journal? You or your colleagues should be aware of the journal as a publication they would read, publish in or cite regularly.