which year is a check taxable if issued in one year and cashed the next year

If you received a check in 2025 that was issued in 2024 it is taxed in 2025. 

A check for what?  Usually when it is issued.  

If an employer, a client or a financial institution issues a check to you in 2024, they are going to include that amount on the tax document--like your W-2 or whatever kind of 1099--- for 2024, so the amounts you enter on your 2024 tax return have to match what is on the tax documents you receive.    Whether you wait to cash the check is irrelevant.