svwMB's were not all postwar French. Law's Backbone of the Wehrmacht was incorrect in this regard. Signs of French completion/use include: only the last 3 digits of the serial number are present on the bolt flat (serial number suffix letter is not on bolt flat), numbered components that would not have been numbered by the Germans--usually bands, floorplate, and triggerguard, French stacking rod screwed into the top of the bayonet mount delete sheetmetal, circle on that sheetmetal plate where a stacking rod once was that has been replaced by a simple screw (i.e. someone trying to make the rifle look less French by ditching the French stacking rod), cutout on the left of the buttstock where a French sling bar resides or used to reside (look for wood filler and nonsense here deposited by someone attempting to make a French stock look less French), French star firing proof, all blued metal on a byf 45/svw 45/svw MB rifle (you can pretty much count on finding at least some phosphated parts on a 1945 assembled rifle), "e" or later serial number suffix on an svwMB coded rifle (the very last known German assembled rifles are in the "d" serial number block). These are some hints but I may have missed an item or two.
During 1945 it appears that Mauser Oberndorf stopped attempting to correct flaws on rifles that were rejected by inspectors. It appears that they came to the conclusion that it made more sense (at that time) to concentrate efforts on regular production. If a rifle was rejected, they simply set it aside. When the French came along, they took control of the rejected rifles along with many parts. They fixed the rejected rifles and continued production where the Germans left off. Therefore there is not a clean break where German rifles stop and French rifles start. You will find French rifles interspersed throughout the 1945 production serial numbers along with a few earlier dated rifles. Take a look at the stickies saved up top for more detailed information (esp. the Mauser Oberndorf serial number study).
Post some more details when you can as we are always interested in hearing more. If you can, post the rifle's information in the serial number thread.