We will need some clarification to answer this question. When you say you "checked for 2018 taxes in the IRS website", did you mean did you check for the 2018 taxes filed or your wage and income information? If certain requirements were met, it could be possible that your ex could have filed as "Married Filing Separately" and hence you do not see your information. Again, we need more clarification to be able to answer this question more accurately.

The IRS will only provide information via transcript relevant to your Social Security number - even if you are filing a tax return with the Married Joint filing status. So if the 2018 income tax return is showing on the IRS Account Transcript as being filed under your Social Security number, then you can rest assured that a tax return was indeed filed. However - you both are obligated under penalty of perjury to sign a married/joint tax return. If you did not sign the 2018 tax forms and yet the 2018 tax forms were processed by the IRS then someone else (probably the X) would have had to sign your name to the tax forms. Forging a taxpayer's signature on income tax forms is fraudulent behavior.

Your husband may have elected to file a separate return which is his right. Economically this is probably not a smart move but it is legal. You need to discuss this with your attorney if you have one.