How to check bsnl validity from other number?

You can't use a USSD code from your phone for another number. It only works from the BSNL SIM itself. The best way is to use the BSNL Selfcare app. You can add their number there to see the details. Another easy trick is to use a payment app like Google Pay. Just go to recharge and enter their BSNL number. The app will usually show you their current plan's expiry date before you have to pay anything.