This is the most reliable test. Send a message; if it shows two gray ticks (delivered) instead of just one (sent), you are no longer blocked.
"WhatsApp Alert: [Contact Name] has unblocked you. You can now send messages." if someone unblocks you on whatsapp
If you want a definitive answer, the only way is to ask them directly (if you’re in contact through other means). This is the most reliable test