Check if your iMessage status says “Delivered” or stays at “Not Delivered”
See if your messages show “Read” in the conversation; if they never switch to “Read,” it may indicate a block (also possible other causes)
Notice if your messages consistently fail to deliver while other contacts still deliver normally
Verify whether you can call them via iMessage/FaceTime; if calls don’t go through, it may indicate a block (also possible other causes)
Try sending a regular SMS (if enabled); if SMS fails or behaves differently, it may indicate a block
Look for changes in their “typing…” or “last seen”/activity indicators (if those features are available to you)
Check whether their contact card photo/name changes or their updates stop appearing (not definitive)
Confirm your number and Apple ID email are correct in your iMessage settings
Ask them directly through another channel to confirm