what to do if client doesn't know amount of EIP?

If you leave it blank the software will calculate the maximum credit allowed as part of the tax return.

When IRS adjusts it for the actual EIP, your client's refund will be less/balance due will be more. Most clients don't respond nicely to IRS notices and adjustments.

Can you calculate what it should have been and use that?

To use the words of a friend "I've got morons on my team".  I added a nice little sheet to mail along with my  organizers this year asking for the amount of their stimulus payments.  Some completely ignore the sheet.  Some will put in only their first payment.  Some will only put in their second payment.  Some will remember one of the payment amounts and then just double it.  And then there are others that went to their dart board and picked some random numbers.  Some of these folks were well below the income limits but yet showed some odd dollar amounts.  I'm not sure if the amounts represent amounts won at the casino (winning at the slot machine does stimulate people) or what they represent.  I'm just thankful that we won't have to go through this again.  Oh, that's right, the bell rang for round 3.

Signing up for the IRS Alerts is a great idea for professional tax preparers:

https://www.irs.gov/newsroom/irs-letters-explain-why-some-2020-recovery-rebate-credits-are-different-than-expected

It isn't clear why they can't check their statements.

They can check "get my payment" and that will tell them if it was issued and to where. Then, you can check the statements.They also can check their Tax Account online; it shows there.