Can bladder cancer can be detected by an ultrasound?

Yes, but...... Ultrasound is not the most sensitive way to look for bladder cancer meaning it can miss early cases and there are other tests that are more sensitive and specific. However, bladder cancers are sometimes seen with ultrasound, especially if they are large.
Yes, but.... This would be an incidental finding on an ultrasound that unfortunately picks up a large likely advanced bladder cancer. Typically some symptoms most commonly blood in urine or pain cause someone to go to the doctor and then a ct scan, urinalysis and cystoscope exam will find or reveal a suspicious area that gets biopsied. The cystoscopic exam directly visualizing bladder lining is best study.