I think it's near impossible to tell how much load generically is needed to damage a firewall, because it's so dependent upon the loading pattern, and the individual design.
How to check for integrity? Have a good hard look at it visually with the cowlings open.
In general however, the firewall carries very little load - the bits I'd worry about getting damaged are the carry through engine mounts and airframe structural components. Even if there are bends or ripples in a firewall, that in itself doesn't compromise it as a firewall - although it may well be indicative of damage elsewhere.
And of course, if in doubt, have it checked by local and suitably qualified maintenance staff.
G