Attachment is quite complex and also cultural/situational. Usually, attached babies smile back at caregivers, they will seek out their main caregiver if upset and expect an appropriate, empathetic response. You’ll often see a baby or toddler laugh or smile in a specific way for a caregiver and have little exchanges between them.
Unattached babies may seek out strangers for comfort or seem unaffected by going off with others. They might not seek caregiver eye contact or reflect back smiles. They may not snuggle in to the caregiver and might look anxious about hugs or close contact, or pull away.
Its all quite dependent, but no single action or deviation from the norm suggests attachment problems, it would be a combination and might appear a bit differently in different situations. It’s often also coupled with neglect or abuse, or perhaps MH difficulties in the parent that have affected ability to bond (eg. if you’re feeling awful inside and struggling to show facial expression).
I would say if you’re present and your babies are happy to associate with others, but they love you and respond to you and you to them (and have done throughout), then attachment is unlikely a cause for concern.
My children haven’t been happy to go to others initially, but have gradually built up bonds with those people that have allowed them to trust them and be left with them. However, I know plenty of babies who took less time than mine. Equally, they respond to me, they smile back at me, they save specific belly laughs for me and dad and siblings, they come straight to me/call me when they need me (or cry and stop crying when picked up), they seek my eye contact and show me things that are important to them (or when older tell me things because they trust me).
My sister (adopted) probably has some attachment issues. She also has ASD, but she was in a hospital for several weeks after birth with no mum (went missing) and no main caregiver. She was named by the nurses after the ward. That lack of a main caregiver meant she never knew who was coming or when, she could cry but wouldn’t be responded to. She barely cried as a baby and took a long time to respond to our facial expressions and smiles. She had to learn that there was any point in connecting with a caregiver. Alternatively, I had a very strong bond with my birth mum and our time together was not reduced prior to adoption. So, I was the other end of the spectrum, I never let my adoptive mum out of my site and believed anyone could leave at any time no matter how strong that bond had been. As toddlers, we both had intense separation anxiety, but my sister internalised hers and stayed very withdrawn, whereas I would be inconsolable.
For most babies, responding lovingly upwards of 50% of the time is enough for secure attachment.