function fraudCheck1(){
funcCheckFraud();
var check = {!fraudCheckFlag};

if(check == false){
    var c = confirm('Mismatch found. Do you want to continue?');
    if(c == true)
        return true;
    else
        return false;
    }
 else{
    return true;
    }
}

<apex:actionfunction name="funcCheckFraud" action="{!checkFraud}" onComplete="alert('After apex method');"  reRender=""/>

public void checkFraud() {
fraudCheckFlag = true;
}