Since ACH works with a bank account, unlike a credit card transaction there is not a way to authorize a charge, or know that funds are available.
By default you just get a response that an ACH transaction was received and that the routing number was for a valid US or Canadian bank.
Some Payment Gateways do offer add-on validation services for ACH / eCheck, however. These services can validate that the bank account number is correct, and even confirm available balance in some cases.
If you are looking to reduce returns and get more information about ACH transactions at the time of the submission, you should contact your payment gateway / payment processor to see if they offer these services.
Small Value Tokenization
It is also possible to process an ACH transaction for a small amount, say $1, in order to create a token in Salesforce and validate the bank account number. This will not confirm the availability of funds beyond the $1 or other amount charged, however.
The $1 transaction could be voided, refunded or credited toward a future balance with your customer, depending on your internal process.