You can't know. You can always use /serverinfo but the information is limited even there. If you see a custom factory then chances are something is modified by it but even with a stock factory it's possible to change gameplay settings on server.cfg and manually.

Hit escape when in game and most if not all modifications will be listed there.
It doesn't go into detail of weapon changes (eg lg 7 instead of 6) but it will show icons for which weapons have been modified ini some way.