It's hard to definitively describe the level of play in the Red River Valley League in 1897, but an educated guess can be made. A few facts to make up our formula:
First, consider that Fargo's team had two former major league players (Jimmy Banning and Josh Reilly) and one future major league player (Deacon Phillippe). Grand Forks and Moorhead each had one future major league player on its 1897 team. Wahpeton-Breckenridge featured no past or future major league players. There were a few stars on each team that also enjoyed long minor league careers, often with higher-level leagues.
With the promising young stars and baseball veterans, however, were a mixture of local players whose baseball resumes were not impressive.
Also consider that the Red River Valley League was considered an informal "farm" league of the 1897 Western League. The Minneapolis Millers, St. Paul Saints, and other Western League teams sent players to the RRVL for parts of the 1897 season. The Western League could fairly be called what we would consider a AAA level league today.
With these facts taken into consideration, it would probably be fair to describe the 1897 RRVL as a A or AA level league - probably something in between these two modern designations. In fact, most of Fargo's minor league teams over the last 125 years would be described similarly in terms of level of play.